Flicker-induced color and form: Interdependencies and relation to stimulation frequency and phase

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Abstract

Our understanding of human visual perception generally rests on the assumption that conscious visual states represent the interaction of spatial structures in the environment and our nervous system. This assumption is questioned by circumstances where conscious visual states can be triggered by external stimulation which is not primarily spatially defined. Here, subjective colors and forms are evoked by flickering light while the precise nature of those experiences varies over flicker frequency and phase. What’s more, the occurrence of one subjective experience appears to be associated with the occurrence of others. While these data indicate that conscious visual experience may be evoked directly by particular variations in the flow of spatially unstructured light over time, it must be assumed that the systems responsible are essentially temporal in character and capable of representing a variety of visual forms and colors, coded in different frequencies or at different phases of the same processing rhythm.

Introduction

Contemporary perspectives hold conscious experience as an emergent property arising from synchronous oscillations at widely dispersed brain areas (Tononi & Edelman, 2002). The precise role of neuronal synchronization for the conscious perceptual representation of visual space remains unknown, while evaluation of whether or not the mechanisms responsible for conscious content depend upon precise temporal coding remains a problem of considerable complexity. It has been shown, nevertheless, that light sensitive mechanisms may be triggered by appropriate periodic stimulation: for example, color opponent cells are capable of following high-frequency flicker well above heterochromatic fusion frequencies, and combine to generate color fusion (Gur & Snodderly, 1997). Further evidence for the role of temporal factors in the perception of colors has also been brought about by research on the subjective phenomena of color and form perception: in one very early description, Purkinje (1823) described the experience of color and form whilst waving his hand vigorously across his closed eyes while facing direct and bright sunlight. Phenomena of flicker-induced color and form experiences have attracted attention not only in scientific circles, but were also thoroughly investigated and discussed by artists (an example being the “Dream Machine” invented by Brion Gysin (Geiger, 2003)) and writers (e.g., Burroughs, 1987).

Subjective color has mostly been studied by using the Benham disk or similar stimuli (Benham, 1895, Campenhausen and Schramme, 1995, Cohen and Gordon, 1949, Fechner, 1838). The Benham disk is composed of a black sector on one half of the disk and white sectors, containing black arcs at different radii from the center of the disk, on the second half. When this disk is spun at frequencies of between 5 and 10 Hz, the different arcs appear with different colors. Counter-clockwise rotation of the disk causes a reversal of the colors on the disk. The role of spatial factors for the apperception of subjective colors in terms of lateral inhibitory effects induced by adjacent parts of the Benham disk is still under debate, and whereas Festinger, Allyn, and White (1971) report solely temporally induced subjective colors, Jarvis’ (1977) failure to replicate these findings suggests the necessity for spatial interactions as important determinant for the appearance of subjective colors.

Besides studies using Benham disks and related stimulations, there are a number of investigations studying not only subjective experiences of color but also subjective forms. Smythies (1959) describes in detail the subjective patterns and forms arising during intermittent stimulation in the visual field. He showed that even though subjective patterns differ in quite specific details from individual to individual, they are similar enough to allow classification. The patterns described in his study consisted of various radial patterns, patterns comprising straight lines on a square base, herringbone and honeycomb patterns, curvilinear patterns, and other complex mosaics. While binocular stimulation yields clearer subjective patterns, the patterns become more fine-grained with increasing stimulation frequency (Smythies compared stimulation frequencies of 6, 12, and 18 Hz). In a very similar vein, the work of Smythies corresponds with that of Knoll and Kugler (1959) who also showed subjective forms to be brought about through periodic electrical stimulation or by means of photic driving, i.e., as a function of exposure to high-frequency flicker. More recently, during an electroencephalographic (EEG) study of the cortical response to flickering light, Herrmann (2001) observed that most of his subjects reported forms (stars or stripes) and colors (blue, red, or purple). From a reexamination of Hermann’s data, Herrmann and Elliott (2001) described a range of color (red, blue, and purple) and form (lines, honeycombs, and tunnels) experiences which appear to be confined to certain ranges of flicker frequencies within approximately the lower 40% of 1–100 Hz stimulation frequencies examined by Herrmann.

Recent attempts to model subjective colors were based on the temporal differences of processing between the different color pathways (Courtney and Buchsbaum, 1991, Grunfeld and Spitzer, 1995). Both models took into account the different temporal characteristics of the different cone types, as described by Schnapf, Nunn, Meister, and Baylor (1990) with the M-cones being the fastest to peak (51 ms) and to have the shortest integration time (19 ms), followed by the L-cones (55 and 28 ms), and S-cones (61 and 34 ms). Courtney and Buchsbaum (1991) derived the impulse response functions and nonlinearities of the three most common wavelength selective on-center parvocellular ganglion cell types (L + M−, M + L−, and S + (L + M)−) from the physiological data and took into account a nonlinearity of the S + (L + M)− cells. Grunfeld and Spitzer (1995) developed a model which includes spatial as well as temporal parameters of the spatially- and cone-opponent ganglion cells (L + M−, M + L−, and S  (L + M)). Besides the temporal parameters, these authors suggest the relevance of a so-called rebound response, a common excitatory response to the turning-off of an inhibitory stimulus. In another approach, Stwertka (1993) suggests a dynamical account of self-organization in the brain as the underlying principle of the experience and transformation of subjective forms. Specifically, he states that the phasic synchronization of sets of spatial orientation tuning columns induced by the intermittent light stimulation might result in the conscious experience of features corresponding to the activity of those tuning columns and that the transformations observed in subjective forms correspond to the trajectory of the states of a dynamical system through its phase space.

The colors and forms reported during exposure to temporally modulated light may be called ‘subjective’ in the sense that they concern the experience of an external structure or quality in the absence of structure or quality-related information in the ambient optic array. Subjective experiences of color and form are thus conscious states that appear to occur solely as a function of the way our visual nervous systems respond to temporal modulation and with no obvious spatial references in the external world. If subjective color and form can arise solely as a function of intermittent stimulation, the possibility of describing conscious visual states exclusively in terms of variations in the temporal patterning of stimulation seems highly promising.

The aim of the studies presented here was to investigate the patterns of appearance of subjective colors and forms in more detail. The first experiment aimed to define: (i) the classes of subjective experience (for both color and form), and (ii) the exact frequency ranges over which subjective experiences are received. Of interest in addition were: (iii) what, if any, patterns of co-occurrence there were between these experiences. Two further experiments probed one transient characteristic of a subset of subjective experiences found to be generally reliably obtained in Experiment 1, aiming to measure the onset time of the subjective experience. Subsequent analyses examined the (iv) relationship of onset times with the phase of intermittent stimulation.

Section snippets

Experiment 1

The aim of Experiment 1 was to determine: (i) the type of subjective experiences reported during the stimulation with flickering light, (ii) the range of flicker frequencies over which these experiences were reported, and (iii) the patterns of co-occurrence between those subjective experiences. Besides defining classes of subjective experiences, the exact ranges of frequencies over which these experiences were reported were derived. On the basis of Herrmann and Elliott’s (2001) observations,

Experiments 2 and 3

The tendency for subjective experiences of form and color to be non-uniformly distributed over flicker frequency indicates the existence of a critical bandwidth within which mechanisms responsible for the emergence of conscious visual states may be triggered by the periodic response of mechanisms sensitive to transient contrast changes at particular frequencies.

This suggestion is also supported by the electroencephalographic (EEG) response to flicker, the amplitude of which indicates a strong,

General discussion

The aim of the work presented here was to study subjective colors and forms induced by flickering light. Based upon free reports during stimulation with flickering light, Experiment 1 revealed a number of classes of subjective forms and colors. The ranges of flicker frequencies were calculated over which these subjective experiences are reliably induced while patterns of co-occurrences of these experiences were also calculated. These analyses suggest that while subjective experiences are

Acknowledgments

The authors express thanks to Raul Kompass and Cees van Leeuwen for critical comments and advice concerning the work described here, Joseph Glicksohn and an anonymous reviewer for valuable comments on the manuscript, Hermann Müller for his assistance during project formulation and to Bernhard Ostler, Christine Falter, and Katja Obermeier for their help in running the experiments. This research was supported by Deutsche Forschungsgemeinschaft (DFG) project Grants MU 1564/3 and EL 248/2.

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