The Complexity of Neural Responses to Visual Stimuli: On Carruthers's Challenge to Block's Overflow Argument Ned Block's Overflow Argument purports to establish that the neural basis of phenomenal consciousness is independent of the neural basis of access consciousness. In a recent paper, Block's argument has been challenged by Peter Carruthers. Carruthers concedes the truth of one of the argument's key steps, namely, that phenomenal consciousness overflows what is in working memory. At the same time, he rejects the conclusion of the argument by developing an account of this overflow that is alternative to Block's. In this paper, I argue that Carruthers's account does not pose a real threat to the Overflow Argument. The overall plausibility of Carruthers's account rests on the empirical plausibility of a claim concerning global broadcasting which, albeit intuitively plausible in light of a lightly-sketched picture of the impact of attention upon neural matters, he offers no sufficient empirical evidence for. Drawing on some important imaging studies that reveal striking facts about neural responses to visual stimuli, I argue for two intimately related claims: first, that the intuitive plausibility of claims like Carruthers's is not a guarantee of empirical plausibility; second, that as concerns the same claims, strong empirical evidence is needed before confident judgments of empirical plausibility can reasonably be formulated. Keywords: overflow argument; phenomenal consciousness; access consciousness; global broadcasting; neural responses; attention 1. Introduction Ned Block's distinction between phenomenal consciousness and access consciousness (e.g., 1995, 2007a, 2007b) has had a huge impact in the philosophy of mind.1 A mental state is said by Block to be phenomenally conscious if there is something it is like for one to be in it. A mental state is said by Block to be access conscious, instead, if its 1 This article has been accepted for publication in Philosophical Psychology, published by Taylor & Francis. 2 content is broadcast in the global workspace, and is thus accessible to a number of consuming mechanisms: e.g., "mechanisms of reporting, reasoning, evaluating, deciding, and remembering" (2007a, p. 491) (more on access consciousness and global broadcasting in §2). Block's distinction has been the object of extensive debate. While it is widely agreed that phenomenal consciousness and access consciousness are different concepts (Carruthers, 2017), do they also pick out different properties? And if they do pick out different properties, do those properties always co-instantiate? Since the publication of his paper "On a Confusion about a Function of Consciousness" (1995), Block's aim has been to establish that phenomenal consciousness and access consciousness empirically disassociate (Carruthers, 2017). The Overflow Argument (e.g., 2007a, 2007b) represents his major (as well as most debated) attempt to do so. The argument purports to establish that the "neural basis of phenomenology does not include the neural basis of cognitive access to it" (2007a, p. 489) or, equally, that the neural basis of phenomenal consciousness is independent of the neural basis of access consciousness. Block takes the results of George Sperling's experiments (1960) to support the view that more information is phenomenally conscious than can be reported or, equally, that the contents of phenomenal consciousness overflow what can be reported. He then argues that this overflow is best explained by assuming that the neural basis of phenomenal consciousness is independent of the neural basis of access consciousness. The vast majority of Block's critics (e.g., Cohen & Dennett, 2011; Phillips, 2011; Stazicker, 2011) have rejected the conclusion of the Overflow Argument by challenging the overflow claim. In a recent paper, Peter Carruthers (2017) has opted for a different approach. Unlike those critics, Carruthers concedes Block the truth of overflow. He also argues, however, that there is a "better" (2017, p. 65), "and empirically plausible 3 [emphasis added]" explanation of this overflow (2017, p. 67), one that supports the view the neural basis of phenomenal consciousness is not independent of the neural basis of access consciousness. The aim of this paper is to challenge Carruthers's attempt to block the Overflow Argument. More precisely, it is to argue that the former does not pose a real threat to the latter. The overall plausibility of Carruthers's account of overflow rests on the empirical plausibility of a claim concerning global broadcasting which, albeit intuitively plausible in light of a lightly-sketched picture of the impact of attention upon neural matters, he offers no sufficient empirical evidence for. Drawing on some important imaging studies that reveal striking facts about neural responses to visual stimuli (e.g., Kastner et al., 1998; Scalf et al., 2011), I argue for two intimately related claims: first, that the intuitive plausibility of claims like Carruthers's is not a guarantee of empirical plausibility; second, that as concerns the same claims, strong empirical evidence is needed before confident judgments of empirical plausibility can reasonably be formulated. The upshot, I suggest, is that Carruthers's alternative account of overflow does not threaten the Overflow Argument. Here is the plan. In §2 I present Block's Overflow Argument in detail, and in §3 Carruthers's challenge to it. Next, in §4, I draw on the results of the abovementioned imaging studies to bring to light the complexity of neural responses to visual stimuli – especially to multiple simultaneously presented stimuli.2 In §5, I then argue that the complexity of neural responses to visual stimuli has some important implications for attempts to assess the empirical plausibility of certain claims about global broadcasting. 2 For brevity, in what follows, I shall often drop the "simultaneously presented" qualification and speak of multiple stimuli alone. 4 Last, in §6, I explain how those implications enable us to disarm Carruthers's challenge to Block's argument. The conclusion I draw is twofold: first, the question whether the neural basis of access consciousness is included in the neural basis of phenomenal consciousness remains far from settled. Second, and more broadly, any future attempt to determine the empirical plausibility of certain claims about global broadcasting will need to attend carefully to the delicate and subtle empirical evidence that bears heavily upon the issue. 2. The Overflow Argument In this section, I present Block's Overflow Argument, following (at least in part) Carruthers's (2017) reconstruction of it. The argument proceeds in two main steps. Drawing on the work of Sperling (1960), the first step purports to establish that more information is phenomenally conscious than is in working memory. From there, the second step concludes that the "neural basis of phenomenology does not include the neural basis of cognitive access to it" (Block, 2007a, p. 489) or, equally, that the neural basis of phenomenal consciousness is independent of the neural basis of access consciousness. Clarifications are already in order – what is working memory? This can be defined as the active storage of information in an accessible form (Baddeley, 2007; Cowan, 2005). Working memory is an active form of memory in that its contents need to be actively sustained. Information retained in working memory is accessible in that it can be used in a number of complex cognitive tasks. To give some examples, we use working memory when doing mental arithmetic or when, in an experimental setting, we are asked to report the contents of a display after stimulus offset. It is widely accepted among cognitive scientists that the capacity of working memory is limited to about three 5 or four "items" (Cowan, 2001). To a first approximation, an item can be thought of as information about an integrated object; that is, for example, information about a shape that specifies its orientation, color, and identity (Luck & Vogel, 1997). Back to the Overflow Argument. In his whole report paradigm, Sperling presented participants with arrays of letters – 3x4 arrays, for example – for 50 milliseconds. After stimulus offset, participants were then required to verbally list all the letters in the array. Although they believed that they had seen all or most of the letters quite clearly, they could only report about four on average. In a second experiment – the partial report paradigm – participants were presented with similar arrays of letters. This time, however, they were only required to give a partial report of the contents of the array. 150 milliseconds after stimulus offset, a randomly chosen row was cued by sounding a tone: a high tone for the upper row, a medium tone for the middle row, and a low tone for the lower row. According to the instructions provided prior to stimulus onset, participants had to report the letters from the cued row. Sperling found that, in each trial, participants were able to accurately report most of the letters from the cued row – typically three out of four letters. Figure 1. Typical array of letters used in the Sperling experiments. From Phillips (2011) Block thinks that the results of the partial report paradigm support the view that (P1) participants are phenomenally conscious of all or most of the presented items in 6 detail. Additionally, given that (P2) the capacity of working memory is limited to three or four items, he concludes that (C1) more information is phenomenally conscious than is in working memory. From there, Block argues that (C1) is best explained by assuming that (C2) the neural basis of phenomenal consciousness is independent of the neural basis of access consciousness. In order to fully appreciate the argument, I still need to explain a number of things. To begin with, Block – and indeed Carruthers – is committed to the Global Workspace Model of access consciousness (e.g., Baars, 1988; Dehaene et al., 2011; Dehaene & Naccache, 2001). According to the model, the architecture of the mind/brain comprises "two main computational spaces" (Dehaene et al., 2011, p. 56): a set of parallel processors whose job is to compute mental representations (e.g., perceptual and quasi-perceptual representations), and a global workspace. The latter is a system that enables processors to "communicate" or exchange information with one another. At any one time, a number of processors compete or cooperate to broadcast information in the global workspace for further processing. Following Block (2007a, p. 491), it will be useful here to distinguish between supplying and consuming processors. Supplying processors (e.g., processors that compute perceptual representations) broadcast information in the global workspace. Consuming processors (e.g., processors that compute mental representations for deliberation and report) instead, take that information as input and process it further. According to the Global Workspace Model, access consciousness is identical to global broadcasting. More precisely, the idea is that an access conscious representation is a representation whose content is globally broadcast, and hence accessible to the consuming processors connected to the workspace. 7 One more thing to note concerns Block's view on the relationship between the global workspace and working memory. Block assumes that the former is identical to the latter (e.g., 2007b, p. 539, 2007a, p. 491). Along with what I said in the paragraph immediately above, this entails a number of things. First, to say that (P2) the capacity of working memory is limited to three or four items, is to say that (P2A) the capacity of the global workspace, and hence of access consciousness, is limited to three or four items. Second, the claim that (C1) more information is phenomenally conscious than is in working memory, is in effect identical to the claim that (C1A) more information is phenomenally conscious than is in the global workspace, and hence than is access conscious. Last, to say that (C2) the neural basis of phenomenal consciousness is independent of the neural basis of access consciousness, is equivalent to saying that (C2A) the neural basis of phenomenal consciousness is independent of the neural basis of global broadcasting, and hence of the neural basis of working memory. For the sake of clarity, it will be useful here to reconstruct the first step of the Overflow Argument – the step from (P1) to (C1) – as follows: (P1A) Information about all or most of the twelve characters is phenomenally conscious; (P2A) The capacity of the global workspace, and thus of access consciousness, is limited to three or four items; Thus, (C1A) More information is phenomenally conscious than is in the global workspace, and hence than is access conscious. What about the second step of the argument – the step from (C1) to (C2)? To appreciate this step, I still need to say a few words about some of the neural and functional events that are thought to be at play in visual perception. When a stimulus is 8 presented in one's visual field, it causes neural activity in the occipito-temporal areas of the brain (e.g., Dehaene & Changeux, 2004; Lamme & Roelfsema, 2000). There is evidence that when one reports seeing the stimulus, attention to the stimulus boosts occipito-temporal activity, causing it to trigger activation in more frontal areas of the brain, such as the prefrontal cortices, anterior cingulate, and parietal areas (e.g., Dehaene et al., 2006; Lamme, 2004). When one does not report seeing the stimulus, instead, neural activity remains confined to the occipito-temporal areas. This activity, nonetheless, can be almost as strong as activity that because of the boost received by attention, triggers activation in more frontal regions (e.g., Dehaene et al., 2006; Lamme, 2004). We are now in a position to return to the second step of the Overflow Argument, namely, the step from (C1) to (C2). According to the Global Workspace Model, the spreading of neural activity from the occipito-temporal areas to more frontal areas is the neural basis of global broadcasting. More precisely, it is the neural basis of the global broadcasting of the information that this activity carries, and hence of access consciousness.3 Block agrees that the spreading of neural activity to more frontal areas is the neural basis of global broadcasting, and hence of access consciousness. However, he also argues as follows. Let us assume that strong neural activity in the occipitotemporal areas of the brain can be (and is) phenomenally conscious independently of its 3 It is normally assumed by neuroscientists and philosophers alike that at least some kinds of neural activity carry information/have content. In the case of neural activity caused by the presentation of a stimulus, the former is thought to carry information about the stimulus. Although neuroscientists do not always intend "content" or "information" in the same way as philosophers – that is, in a semantical sense – in the present debate it is generally assumed that they do (Wu, 2018). 9 spreading to more frontal areas, and thus of access consciousness. We will then have a mechanism that explains how (C1) more information is phenomenally conscious than is in working memory. On this basis, Block concludes that (C1) is best explained by assuming (C2), namely, that the neural basis of phenomenal consciousness is independent of the neural basis of access consciousness.4 3. Carruthers's Response to the Overflow Argument In this section I present Carruthers's attempt to block the Overflow Argument. As anticipated, Carruthers grants Block the truth of overflow. That is, he grants that (C1) more information is phenomenally conscious than is in working memory. Unlike Block, however, he also thinks that this fact is not best explained by the claim that (C2) the neural basis of phenomenal consciousness is independent of the neural basis of access consciousness. In his view, there is a "better" (2017, p. 65) "and empirically plausible" (2017, p. 67) explanation of (C1), one that supports the view that the neural basis of phenomenal consciousness is not independent of the neural basis of access consciousness. To make his point, Carruthers puts forward five related theses, which he suggests are all widely accepted among cognitive scientists (2017, p. 68): 4 One might wonder whether the truth of (C2) is a necessary condition for the truth of the claim that phenomenal consciousness and access consciousness disassociate. Could not phenomenal consciousness and access consciousness come apart but share the same neural basis? This is a very interesting question, one that I would tentatively answer in the negative. For if we understand the neural basis of a mental property P to be sufficient for the tokening of P, then how could phenomenal consciousness and access consciousness have the same neural basis and yet disassociate? The issue is certainly worthy of closer inspection, and I am thankful to an anonymous reviewer for bringing it to my attention. 10 (1) "[A]ttentional signals directed at representations in sensory regions of the brain are a necessary (and, with other factors, sufficient) condition for those representations to be globally broadcast"; (2) "attention is a limited resource: only so much information can be attended to at any one time"; (3) "the effect of attentional signals is to boost the neural activity underlying the targeted representations"; (4) "working memory uses the same attentional network to sustain previouslypresented sensory representations in the global workspace"; (5) "global broadcasting takes place when some sort of threshold of neural activity is reached". Before expanding on Carruthers's account of (C1), allow me to clarify what he means by "sensory regions" – this is important, both for present and later purposes. Carruthers takes the phrase to refer to mid-level, as opposed to low and high-level, visual areas. There is some debate as to which visual areas classify as mid-level and which as lowlevel. Among the mid-level areas, however, Carruthers (2015) would certainly include areas V2, V3, V3A, V4, and V5 (see also Prinz, 2012). In his own words, these areas "receive input from V1 and process the motion, color, and form of a stimulus, but without yet conceptualizing or categorizing it" (2015, p. 14). It is precisely mid-level areas that are thought to process the contents that can enter the global workspace (e.g., Carruthers, 2015; Prinz, 2012). And it is also activity in the very same areas that Block thinks can be phenomenally conscious in the absence of global broadcasting. Two more things to clarify concern Carruthers's understanding of access consciousness and his understanding of the relationship between working memory and the global workspace. Carruthers notes that Block (1995) characterizes access 11 consciousness dispositionally: a mental representation is access conscious "in the sense that [its content] is available (counter-factually) to systems for forming memories, for generating affective reactions, for planning, and for verbal report" (Carruthers, 2017, p. 65). Carruthers also notes, however, that in his most recent writings (e.g., 2007a) Block has been leaning towards a categorical understanding of access consciousness: a mental representation is access conscious in the sense that its content is actually globally broadcast, and is thus accessible to the consuming systems connected to the workspace. Importantly, the emphasis here is on "actually globally broadcast", rather than on "accessible to the consuming systems". It is the categorical understanding of access consciousness that Carruthers takes Block to adopt in his Overflow Argument. And it is the same categorical understanding that Carruthers assumes in his response to Block's argument.5 Now for Carruthers's understanding of the relationship between working memory and the global workspace. Like Block, Carruthers thinks that the two are intimately related. Unlike Block, however, he also thinks that they are not identical. On the basis of thesis (4), but also on the basis of his treatment of the issue elsewhere (2015), we can understand Carruthers's view on the matter as the conjunction of two claims. First, the contents of working memory are stored in the workspace. This is to say that, in effect, working memory is parasitic on the workspace. Second, the contents of working memory are normally only a proper subset of the contents of the workspace: 5 Whether access consciousness should be characterized in dispositional or categorical terms is an issue that, with few exceptions (see below, for example), has not received the attention that it deserves. While discussing the issue is beyond the scope of this paper, the interested reader will find a stimulating discussion of it in Stoljar (2019) and in Block's (2019) reply to Stoljar. 12 it is only those contents that are actively sustained (rather than merely broadcast) in the workspace that qualify as working memory contents. Note that, as Carruthers explains, "the asymmetry envisaged here is diachronic" (2015, p. 84). That is, it is normally the case that, over a period of time T, the amount of information that enters the workspace is larger than the amount that enters working memory. Figure 2. From left to right, graphical illustrations of Carruthers's and Block's take on the relationship between working memory and the global workspace. Carruthers takes working memory to be intimately related to, albeit also different from, the global workspace. While the contents of working memory are stored in the workspace, they are normally only a proper subset of the contents of the latter. Block, on the other hand, takes working memory to be identical to the global workspace. The left figure is redrawn from Carruthers (2011, p. 49). We are now ready to introduce Carruthers's response to the Overflow Argument. With the above five theses in place, Carruthers notes that the reason why Block's argument fails should be fairly easy to see. The thought is that the amount of attention needed to actively sustain the contents of a representation in working memory is much larger than the amount required for the same contents to be globally broadcast. In the latter case, neural activity will already be strong due to the presence of the stimuli. This suggests that less attention will be needed in order to boost that activity over the threshold for global broadcasting (2017, p. 68). The same cannot be said with respect to sustaining a representation in working memory, however. For in that case attention will have to do its work all by itself. That is, it will have to sustain neural 13 activity beyond the global broadcasting threshold in the absence of bottom-up stimulation. As a consequence, claims Carruthers, "greater richness and detail may be broadcast in perception than can be sustained in working memory thereafter" (2017, p. 68). In this way, Carruthers can grant that (P1) participants in the Sperling experiments are phenomenally conscious of all or most of the presented items in detail; explain why (P2) the capacity of working memory is limited to three or four items; and vindicate the claim that (C1) more information is phenomenally conscious than is in working memory. At the same time, however, he can reject the conclusion of Block's argument, namely, that (C2) the neural basis of phenomenal consciousness is independent of the neural basis of access consciousness. When participants are presented with an array of letters, they distribute their attention "evenly over the entire display" (2017, p. 69). Because exogenously caused neural activity is already high, "attention [...] may be sufficient to boost the neural activity caused by those stimuli over the threshold for global broadcasting." This, in turn, results in phenomenal (and access) consciousness of "most of those items in identity-defining detail" (2017, p. 68). At this stage – call it T1 – information about the array is broadcast in the workspace, but not yet retained in working memory. After stimulus offset – call this stage T2 – exogenously caused neural activity will gradually drop. In order to report the letters that they have seen, participants will now have to hold the relevant information in working memory. That is, they will now have to actively sustain the relevant neural activity, "holding [it] far enough above baseline for global broadcasting to continue to take place" (2017, p. 68). Crucially, however, in the absence of bottom-up stimulation, attentional resources will not suffice to sustain a detailed representation of the array in a broadcast state. Instead, all 14 attentional resources will now have to be focused on a much smaller number of characters – three or four – and withdrawn from the others. As a result, three or four is the number of letters that participants can normally report. On this account, the contrast that Block draws between rich phenomenal consciousness and "content-limited" access consciousness is rather a contrast between "rich stimulus-driven perception (which is both access-conscious and phenomenally conscious) and limited-content working memory (which is likewise both accessconscious and phenomenally conscious)" (2017, p. 69). As a consequence, concludes Carruthers, the step from (C1) to (C2) is unwarranted: the overflow of working memory by phenomenal consciousness provides no grounds for arguing that (C2) the neural basis of phenomenal consciousness is independent of the neural basis of access consciousness. 4. The Complexity of Neural Responses to Visual Stimuli In this section, I present and discuss some important imaging studies relating to neural responses to visual stimuli. My aim here is to bring out the complexity of such responses, complexity which is due to their dependence on a large variety of factors; e.g., factors relating to presentation and attention conditions. Let me start by clarifying how this fits into my overall discussion. One of the key steps of Carruthers's argument is "that attention distributed over all twelve stimuli may be sufficient to boost the neural activity caused by those stimuli over the threshold for global broadcasting", and that this results in one's being phenomenally (and access) conscious of "most of those items in identity-defining detail" (2017, p. 68). In light of a lightly-sketched picture of the impact of attention upon neural responses, like the one offered by Carruthers, this seems like an intuitively 15 plausible claim: neural activity caused by the stimuli is already strong, and attentional resources distributed over the stimuli may thus be sufficient to trigger the global broadcasting of rich information about them. Call claims to the effect that the distribution of attention over multiple stimuli results in the global broadcasting of rich information about the stimuli C-claims. Is the intuitive plausibility of C-claims a guarantee of empirical plausibility? My contention is that it is not. Let us assume that all five theses endorsed by Carruthers are true (see §3 above). Let us also concede Carruthers that in the Sperling experiments participants distribute their attention evenly over the array. Drawing on the imaging studies that I hinted at above, in the remainder of this paper I argue that first, the intuitive plausibility of Cclaims is not a guarantee of empirical plausibility; second, that as concerns the same claims, strong empirical evidence is needed before confident judgments of empirical plausibility can reasonably be formulated. The upshot, I suggest, is that Carruthers's alternative account of overflow does not threaten the Overflow Argument. Now for the studies that I have in mind. Using functional magnetic resonance imaging (fMRI), neuroscientists working on attention have provided evidence to the effect that, under certain conditions, the simultaneous presentation of multiple stimuli in the absence of attention results in neural activity in one or more visual areas' being weaker, as compared to when one of the stimuli is presented alone and no attention is present (e.g., Kastner et al., 1998, 2001; Kastner & Ungerleider, 2000; Scalf & Beck, 2010). 6 Call the simultaneous presentation of multiple stimuli in the absence of 6 Authors like Kastner et al. (1998) construe this seemingly counterintuitive fact as evidence that under certain conditions multiple stimuli compete for representation in visual areas. Whether this is right, however, is irrelevant for present purposes. 16 attention unattended-simultaneous, and the presentation of one stimulus in the absence of attention unattended-sequential. Just how large the difference in neural activity under the two conditions is – and whether there is any difference at all – appears to depend on at least the following factors: the visual area whose activity is being measured; the complexity, size, and number of the stimuli; the location of the stimuli in the visual field; and the distance between the stimuli. Suppose, for example, that four complex stimuli are presented in the upper-right quadrant of the visual field. Suppose, in addition, that the stimuli are presented within a 2°x2° display. The difference between neural activity in V2 under unattendedsimultaneous, on the one hand, and unattended-sequential, on the other, will be much larger than when two of those stimuli are presented in the same area (Kastner et al., 2001; S. Kastner, personal communication, July 12-13, 2018). By contrast, if four stimuli are presented in a display spanning the upper and lower-right quadrants of the visual field, the difference between V2 neural responses under unattended-simultaneous and unattended-sequential will be minimal, if not null (Kastner et al., 2001). Additional evidence relates to one of the effects that directing attention to a single stimulus has on neural activity in the visual areas. Consider the case where multiple stimuli are presented simultaneously in the absence of attention, in such a way that their presentation results in neural activity in some visual area's being weaker under unattended-simultaneous than under unattended-sequential. Directing attention to only one of the stimuli has been found to boost neural responses to the same extent as when attention is directed to the same stimulus presented alone (e.g., Kastner et al., 1998). Let us now close in on an experiment conducted by Paige Scalf et al. (2011). Against the background of the above studies, the authors sought to understand the effects of distributed attention on neural responses in visual area V4. Eight participants 17 were presented with five stimuli in five locations in the upper-right quadrant of the visual field. Each stimulus was centered in each of five squares arranged in a 6.14°X6.14° grid (2011, p. 295).7 A total of ninety-six different stimuli were created by crossing four shapes (circles, squares, triangles, or hearts) with six colors (blue, yellow, green, red, purple, or orange) and four textures (solid, vertical stripes, horizontal stripes, and diagonal stripes). Each block of trials involved the same set of stimuli and the combination of two presentation conditions, sequential and simultaneous, with two attention conditions, attended and unattended. Under sequential presentation each of the five stimuli appeared in isolation and in a random order for 250 milliseconds. Under simultaneous presentation, instead, the five stimuli were presented together for 250 milliseconds. Across all trials, participants were to hold fixation on a "rapid serial visual presentation (RSVP) stream [...] of digits (1-9) and ASCII symbols (%, &, *, #) and a single letter ('a')" (2011, p. 296). In the unattended-sequential and unattendedsimultaneous conditions, participants were to monitor the stream for the "a". In the attended-sequential and attended-simultaneous conditions, instead, they were to "[search] for a color/shape/texture conjunction in any of the five locations" (2011, p. 295). 7 Information about grid size is due to Diane Beck (personal communication, July 11, 2018). 18 Figure 3. Sequential and simultaneous presentation of the five stimuli. From Scalf et al. (2011). Using fMRI, Scalf and colleagues obtained some interesting results. First, in line with the results obtained in a previous study (Scalf & Beck, 2010), V4 neural activity under unattended-simultaneous was found to be weaker than under unattendedsequential. Second, consistently with the results reported earlier on, V4 neural activity under attended-sequential was observed to be stronger than under unattendedsequential. As concerns the main aim of the experiments – to explore the effects of distributed attention on V4 neural activity – activity under attended-simultaneous was found to be stronger than under unattended-simultaneous. Crucially, however, activity under attended-simultaneous was also found to be weaker than under attendedsequential. In other words, although distributing attention over multiple stimuli was found to boost V4 neural activity, it never boosted it as much as did directing attention to one stimulus. 19 Figure 4. From left to right, V4 neural activity under unattended-sequential, unattended-simultaneous, attended-sequential, and attended-simultaneous. From Scalf et al. (2011). The studies discussed thus far lend support to the third of Carruthers's theses above; namely, that (3) "the effect of attentional signals is to boost the neural activity underlying the targeted representations" (2017, p. 68). At the same time, they also bring out the complexity of neural responses to visual stimuli – especially to multiple simultaneously presented ones – as a direct consequence of their dependence on a large variety of factors. But how is this relevant for my challenge to Carruthers's response to the Overflow Argument? The idea is that this complexity has some important implications for attempts to determine the empirical plausibility of C-claims. To reveal these implications, in the next section, I start by arguing that in the Scalf et al. experiment the global broadcasting of V4 information under attended-simultaneous does not always occur. 5. The Complex Affair of Global Broadcasting It should be uncontroversial that in the Scalf et al. experiment the global broadcasting of rich V4 information occurs under attended-sequential across all (or most) trials: attention is directed to a single stimulus that is presented for a rather long time – 250 milliseconds – and this results in the global broadcasting of information about the 20 stimulus. But does the global broadcasting of rich V4 information also occur under attended-simultaneous? More precisely, does the distribution of attention over five stimuli under attended-simultaneous result in the global broadcasting of rich V4 information about them? In light of Carruthers's broad picture of the effects of attention upon neural responses, it would be tempting to say yes. Neural activity caused by the stimuli is already strong, and attention may thus be sufficient to boost that activity beyond the threshold for global broadcasting. In light of the empirical findings discussed in §4, however, things may be more problematic than they seem. One of the main issues here is that V4 neural activity under unattendedsimultaneous is much weaker than under unattended-sequential. This suggests that significantly more attentional resources will be needed to boost V4 activity caused by multiple stimuli over the broadcasting threshold than activity caused by a single stimulus. Another potential issue arises if we consider that attention is a limited resource. In light of this, the amount of attention allocated to each of the five stimuli will presumably be much smaller than the amount allocated to a single stimulus. On the basis of these two points, we can formulate the following hypothesis: in at least some (if not, as we will see, in several) trials, the global broadcasting of V4 information about multiple simultaneously presented stimuli does not occur.8 8 Why not all trials? For although Scalf et al. speak as if V4 neural responses under unattendedsimultaneous were always much weaker than under unattended-sequential, what their study actually shows is that V4 neural responses under the former condition tend to be, in a statistically significant way, much weaker than under the latter condition. This suggests that we need to remain open to the possibility that the difference in activity between the two conditions may sometimes be smaller than what is represented in figure 4. 21 The hypothesis under scrutiny is strongly supported by the results of an additional experiment that Scalf et al. conducted in order to determine the behavioral consequences of their previous findings (2011, pp. 300–301). Based on the results of their V4 imaging studies, the authors predicted that behavioral performance in certain tasks would be significantly worse under attended-simultaneous than under attendedsequential. To test their hypothesis, the authors presented participants with arrays of stimuli whose spatial layout and components were identical to those described earlier on. Unlike before, however, each block of trials involved the combination of sequential and simultaneous presentation conditions with the attended condition alone. Prior to each trial, participants were presented with a specific conjunction of color/shape/texture. The task was "to respond as quickly as possible [by pressing a button] if the conjunction appeared at any point in the display [and a different button] if they did not see the target at any point in the trial" (2011, p. 300). As predicted on the basis of the results of their imaging studies, Scalf and colleagues found that performance under attended-simultaneous was significantly worse than under attendedsequential. In other words, they found that under attended-simultaneous, participants failed to detect the presence or absence of the color/shape/texture conjunction in a significantly larger number of instances than under attended-sequential. What explains this difference in performance? A natural suggestion is that V4 information about the stimuli under attended-simultaneous is not globally broadcast across several trials and is thus not available for participants to guide their responses in the same trials. It is well known that V4 plays a crucial in the representation of color, shapes, and boundaries (Gazzaniga et al., 2014, p. 203). Accordingly, if no V4 information about a stimulus is globally broadcast, no information about any of those features will be made available to mechanisms of reporting, reasoning, deciding, and so 22 on. In this sense, the results obtained by the authors lend further support to our hypothesis: across several trials, the global broadcasting of V4 information under attended-simultaneous does not occur. For my purposes, the empirical plausibility of this hypothesis is important in two ways. To begin with, it clearly shows that as concerns C-claims, intuitively plausibility is not a guarantee of empirical plausibility. As noted earlier on, in light of the lightlysketched picture of the impact of attention on neural responses offered by Carruthers, it would be quite tempting to argue as follows: when participants in the Scalf et al. experiments distribute their attention over five stimuli, rich V4 information about those stimuli is broadcast in the global workspace across all (or most) trials. But now we have good reason to believe that this is not the case. Better, we have good reason to believe that across several trials, the global broadcasting of V4 information about the stimuli does not occur. The same hypothesis can also serve as a platform to advance a second claim that is intimately related to the one above; namely, that as concerns C-claims, confident judgements of empirical plausibility can only be made in the presence of strong empirical evidence. For suppose that some of the factors the strength of neural responses depends upon were altered. Suppose, for example, that the grid used in the Scalf et al. experiments were positioned in a different area of the visual field. What effects would this have on V4 neural responses under attended-simultaneous? In one experiment, Kastner et al. (2001) presented a 6°x6° array of four stimuli in the upperright quadrant of the visual field. Then they presented the same display but in such a way that this spanned two quadrants of a hemifield. While in the former case V3A responses were observed to be significantly stronger under unattended-sequential than 23 under unattended-simultaneous, in the latter case the difference in neural strength between the two conditions was negligible (Kastner et al., 2001, p. 1405). The upshot here I believe is this. Even though, as I have argued, there are several trials of the Scalf et al. experiments where the global broadcasting of V4 information does not occur, it is unlikely that the same considerations would hold if the location of the grid or, say, the distance between the stimuli in the experiments were altered. In the case in point, depending on the location of the grid, there may be series of trials where the global broadcasting of V4 information always occurs, but also series of trials where it never does. What this suggests, in turn, is that assessing the empirical plausibility of C-claims requires careful attention to the delicate and (in some ways surprising) subtle empirical evidence that bears upon the issue – evidence of the sort provided in support of my hypothesis about global broadcasting in the Scalf et al. experiments, for example. One worry may be my argument relies too heavily on the solidity of that hypothesis. My main reason for focusing on the question whether the global broadcasting of V4 information occurred under attended-simultaneous in the Scalf et al. experiments was twofold: to give one a sense of the intricate relationship between neural responses, global broadcasting, and behavioral responses, but also a sense of the sort of empirical evidence that I believe is needed to assess the empirical plausibility of C-claims. Having said that, I believe that we can establish the two main points of my discussion in the absence of that hypothesis. Let me explain. The imaging studies discussed in §4 suggest that the strength of neural responses to multiple stimuli depends on a multiplicity of factors; e.g., the complexity and location of the stimuli, the distance between the stimuli, and so on. Most of these things, in turn, vary from one visual scene to another. But in virtue of the intimate relationship between neural responses and 24 global broadcasting, it seems clear that whether and which sort of information (e.g., information processed by V2, V4, or all mid-level areas) about the stimuli is globally broadcast strongly depends on the specific features of each visual scene. This suggests two things. First, as before, the intuitive plausibility of C-claims is not a guarantee of empirical plausibility. As noted on more than one occasion, on the basis of Carruthers's broad picture of the effects of attention on neural responses, it may be tempting to argue that when attention is evenly distributed over multiple stimuli, attentional resources may be sufficient to trigger the global broadcasting of rich information about them. But in light of the fact that the occurrence and extent of global broadcasting are strongly dependent on the specific features of a visual scene, the intuitive plausibility of claims of this sort is no guarantee of empirical plausibility. Second, and for exactly the same reason, making sensible assessments of the empirical plausibility of these claims requires strong empirical evidence. I take it to have convincingly argued that the complexity of neural responses to multiple stimuli has some strong implications for attempts to determine the empirical plausibility of C-claims. The question to address now is how this is relevant for Carruthers's response to Block's Overflow Argument. This is what I intend to do below. 6. Back to Overflow In his attempt to offer a "better" (2017, p. 65) and "empirically plausible [emphasis added]" (2017, p. 67) explanation of the overflow of phenomenal consciousness by working memory, Carruthers argues that when participants in the Sperling experiments are presented with an array of letters, "attention is distributed evenly over the entire display" (2017, p. 69) – the truth of this claim, I have conceded at the beginning of §4. 25 Furthermore, he also claims that "attention distributed over all twelve stimuli may be sufficient to boost neural activity caused by those stimuli over the threshold for global broadcasting" (2017, p. 68), and that this results in one's being phenomenally – and access – conscious of "most of those items in identity-defining detail" (2017, p. 68). In light of my extensive discussion about neural responses to visual stimuli and global broadcasting, it should now be clear why Carruthers's alternative account of overflow is not a real threat to Block's. That the distribution of attention over the Sperling array may be sufficient to trigger the global broadcasting of rich information about it is, as noted, an intuitively plausible idea: neural activity caused by the stimuli is already strong, and attentional resources distributed over the stimuli may thus be sufficient to boost it over the global broadcasting threshold. Because of the experimentally-uncovered complexity of neural responses to multiple visual stimuli, however, the empirical plausibility of this claim is not at all obvious. For all we know, it is entirely possible that distributing one's attention over the Sperling array may never boost neural responses in any of the mid-level sensory areas over the threshold for global broadcasting. Or perhaps it will boost neural responses beyond the global broadcasting threshold in some instances but not others. Or yet maybe it will boost neural responses over that threshold in V2 and V3, but not in V4. The moral of the story, I believe, is this: the empirical plausibility of Carruthers's claim about global broadcasting in the Sperling experiments cannot be established on the basis of its mere intuitive plausibility. Rather, the rich experimental detail that has been uncovered in relation to attention and visual neurology shows that here – and in many other cases too, we might suspect – strong empirical evidence is needed before confident judgements of empirical plausibility can reasonably be formulated. As Carruthers provides no such evidence for his claim, his account of the 26 overflow of working memory by phenomenal consciousness does not threaten the Overflow Argument. It is worth stressing that my observations have consequences that extend beyond Carruthers's own argument. In their own attempt to question the soundness of Block's Overflow Argument, for example, Naccache and Dehaene argue as follows: When subjects [in the Sperling experiments] report seeing "all the letters," we suggest that they distribute their attention globally over the array, and thus are only able to determine its approximate numerosity and "letterhood"; our model predicts that only this approximate content, not the detailed letter identities, accesses a frontoparietal global neuronal workspace (Naccache & Dehaene, 2007, p. 519). Naccache and Dehaene's argumentative strategy is different from Carruthers's. Unlike the latter, the authors contend that the participants' belief to have seen all or most of the letters in great detail should not be taken at face value. Rather, they suggest that when participants distribute their attention over the array, the only information that is globally broadcast is information about the array's "approximate numerosity and 'letterhood'". But although Naccache and Dehaene say that this is what the Global Workspace Model predicts, no evidence of any form is offered in support of their claim. Thus, for exactly the same reasons why we should be skeptical of Carruthers's own suggestion, the value of Naccache and Dehaene's claim is easily put into question: in the absence of strong empirical evidence, there are no grounds here to believe that the distribution of attention over the Sperling array results in the global broadcasting of information about the array's "approximate numerosity and 'letterhood'". 8. Concluding Remarks In this paper, I have argued that Carruthers's alternative account of overflow does not threaten Block's Overflow Argument. The overall plausibility of Carruthers's account 27 rests on the empirical plausibility of a claim that has some intuitive plausibility, but for which he does not provide sufficient empirical support. Drawing on a number of imaging studies to reveal the complexity of neural responses to visual stimuli, I have argued for two intimately related claims: first, that the intuitive plausibility of claims like Carruthers's – what I have called C-claims – is not a guarantee of empirical plausibility; second, that as concerns the same claims, confident judgements of empirical plausibility can only be made in the presence of strong empirical evidence. Of course, my discussion will raise a number of questions that I am unable to address herein. For example, suppose we found evidence to the effect that the distribution of attention over the Sperling array triggered the global broadcasting of a detailed representation of four or five characters, thereby refuting Carruthers's alternative account of overflow. This would leave us with the question whether detailed information about most characters was phenomenally conscious in the absence of global broadcasting.9 Consider also the issue of the relationship between working memory and the global workspace. While Block assumes that the two are identical, Carruthers thinks that they are intimately related but not identical. Which view, if any, is on the right track? This question is hugely important, not just in relation to the Overflow Argument, but also as concerns our understanding of the notion of access consciousness more generally. Both issue notwithstanding, two things seem clear enough: First, the question whether the neural basis of phenomenal consciousness is independent of the neural basis of access consciousness remains far from settled. Second, and more broadly, any future attempt to establish the empirical plausibility of C-claims – but also of claims 9 I am thankful to an anonymous reviewer of this paper for bringing this issue to my attention. 28 along the lines of Naccache and Dehaene's – will need to attend carefully to the delicate and subtle empirical evidence that bears heavily upon the issue.10 References Baars, B. J. (1988). A cognitive theory of consciousness. Cambridge University Press. Baddeley, A. D. (2007). Working memory, thought, and action. Oxford University Press. Block, N. (1995). On a confusion about a function of consciousness. Behavioral and Brain Sciences, 18(2), 227–247. Block, N. (2007a). Consciousness, accessibility, and the mesh between psychology and neuroscience. 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