Flicker-light induced visual percepts: Frequency dependence and specificity of whole percepts and percept features Carsten Allefeld ∗ , Peter P ¨ utz, Kristina Kastner, Jiˇ r´ ı Wackermann Department of Empirical and Analytical Psychophysics, Institute for Frontier Areas of Psychology and Mental Health, Wilhelmstraße 3a, 79098 Freiburg, Germany preprint, accepted for publication in Consciousness and Cognition doi:10.1016/j.concog.2010.10.026 Abstract Homogeneous flickering light induces color and form hallucinations in human observers. Despite a long scientific his- tory of the phenomenon, still little is known about the dependence of flicker-light induced impressions on the flicker frequency. We investigate this question using Ganzfeld (whole-field) stimulation and an experimental paradigm that combines a continuous frequency scan over the range 1–50 Hz with a focus on re-occurring, whole percepts. On the single-subject level, we find a high degree of frequency stability of re-occurring individual percepts across experimen- tal passes. To generalize findings across subjects, we apply two rating systems, (1) a set of complex percept classes derived from subjects’ verbal reports and sketches, and (2) a systematic enumeration of elementary percept features, and determine the distribution of percept occurrences over flicker frequency for each of the percept categories. For many categories, we find distributions that significantly deviate from a reference distribution. We observe a stronger frequency specificity for complex percept classes than elementary percept features, which points to a possible involve- ment of higher visual cortical areas in their generation. Comparing the similarity relations among percept categories to those among associated frequency profiles, we observe that though particular percepts are preferentially induced by particular frequencies, the flicker frequency does not unambiguously determine the induced percept. This ambiguity suggests that the underlying neurophysiological dynamical process exhibits multistability. Key words: flicker light, Ganzfeld, visual hallucinations, frequency dependence, frequency specificity, phenomenology 1. Introduction Stimulation with spatially uniform, colorless, flicker- ing light induces in human observers hallucinatory per- ceptions of form and color. This phenomenon, which is known since the early 19th century, has been investigated by a number of researchers. However, until today rela- tively little is known about the relation between the sub- jective percepts and the most important property of the stimulation: the flicker frequency. This relation was studied in detail only recently by Becker and Elliott (2006), who determined the frequency profiles associated with a set of elementary visual fea- tures. The present paper continues that line of investi- gation and expands on their work in the following way: we introduce a stimulation technique aimed at eliciting a ∗ Corresponding author. Current affiliation: Bernstein Center for Computational Neuroscience and Charit´ e, Berlin, Germany Email address: carsten.allefeld@bccn-berlin.de (Carsten Allefeld) strong and reliable response to the flicker light; we use an experimental paradigm focusing on re-occurring visual percepts; and we apply methods of data analysis tailored to assess the stability and specificity of the observed fre- quency dependence of flicker-induced phenomena. The following section gives an overview of the history of investigations of flicker-induced and related phenom- ena. The aims and the design of our contribution are de- scribed in detail in Sec. 3 and the experimental protocol and data analysis procedures in Sec. 4. Following a report of our results in Sec. 5, their implications with respect to the underlying neural processes and the relation to previ- ous findings are discussed in Sec. 6. 2. Flicker-induced and related phenomena 2.1. Observations The first scientific report on flicker-light induced pat- terns was given by Purkinje (1819, Sec. I), in his study of various visual phenomena occurring under different