Inertia and memory in ambiguous visual perception.

Perceptual multistability during ambiguous visual perception is an important clue to neural dynamics. We examined perceptual switching during ambiguous depth perception using a Necker cube stimulus, and also during binocular rivalry. Analysis of perceptual switching time series using variance-sample size analysis, spectral analysis and time series shuffling shows that switching times behave as a 1/f noise and possess very long range correlations. The long memory feature contrasts sharply with the traditional satiation models of multistability, where the memory is not incorporated, as well as with recently published models of multistability and neural processing, where memory is excluded. On the other hand, the long memory feature favors the concept of "dynamic core" or coalition of neurons, where neurons form transient coalitions. Perceptual switching then corresponds to replacement of one coalition of neurons by another. The inertia and memory measures the stability of a coalition: a strong and stable coalition has to be won over by another similarly strong and stable coalition, resulting in long switching times. The complicated transient dynamics of competing coalitions of neurons may be addressable using a combination of functional imaging, measurement of frequency-tagged magnetoencephalography and frequency-tagged encephalography, simultaneous recordings of groups of neurons in many areas of the brain, and concepts from statistical mechanics and nonlinear dynamics theory.

Perception of forbidden colors in retinally stabilized equiluminant images: an indication of softwired cortical color opponency?

In color theory and perceptual practice, two color naming combinations are forbidden-reddish greens and bluish yellows-however, when multicolored images are stabilized on the retina, their borders fade and filling-in mechanisms can create forbidden colors. The sole report of such events found that only some observers saw forbidden colors, while others saw illusory multicolored patterns. We found that when colors were equiluminant, subjects saw reddish greens, bluish yellows, or a multistable spatial color exchange (an entirely novel perceptual phenomena); when the colors were nonequiluminant, subjects saw spurious pattern formation. To make sense of color opponency violations, we created a soft-wired model of cortical color opponency (based on winner-take-all competition) whose opponency can be disabled.

Perception of spatiotemporal random fractals: an extension of colorimetric methods to the study of dynamic texture.

Recent work establishes that static and dynamic natural images have fractal-like l/falpha spatiotemporal spectra. Artifical textures, with randomized phase spectra, and 1/falpha amplitude spectra are also used in studies of texture and noise perception. Influenced by colorimetric principles and motivated by the ubiquity of 1/falpha spatial and temporal image spectra, we treat the spatial and temporal frequency exponents as the dimensions characterizing a dynamic texture space, and we characterize two key attributes of this space, the spatiotemporal appearance map and the spatiotemporal discrimination function (a map of MacAdam-like just-noticeable-difference contours).

Sensorimotor adaptation to violations of temporal contiguity.

Most events are processed by a number of neural pathways. These pathways often differ considerably in processing speed. Thus, coherent perception requires some form of synchronization mechanism. Moreover, this mechanism must be flexible, because neural processing speed changes over the life of an organism. Here we provide behavioral evidence that humans can adapt to a new intersensory temporal relationship (which was artificially produced by delaying visual feedback). The conflict between these results and previous work that failed to find such improvements can be explained by considering the present results as a form of sensorimotor adaptation.

The Leaning Tower of Pisa effect: an illusion mediated by colour, brightness, and motion.

We report a novel, easily observed, and extraordinarily striking optical illusion mediated by interactions of colour, brightness, form, and motion perception--the Leaning Tower of Pisa (LTOP) illusion. Under some circumstances, the perception of orientation of coloured forms is radically altered by rotary movement. We demonstrate that this kinetic effect--easily reproduced with a common record turntable--is optimised by particular colour and brightness differences between foreground and background with an illusory tilt of 8 degrees and more. The described illusions can be easily studied at home by downloading the colour figures from www.perceptionweb.com/perc1000/ditzinger, printing them on a common colour printer and placing them on a rotating record turntable.

Very short-term visual memory via reverberation: a role for the cortico-thalamic excitatory circuit in temporal filling-in during blinks and saccades?

There is a large projection of neurons from Layer VI of V1 that makes excitatory connections on LGN relay cells. It has been proposed that this circuit is involved in signal processing and thalamic sensitivity regulation. Alternatively, Crick has suggested that the circuit could be a reverberatory loop-a site for very short-term (iconic) visual memory. This hypothesis is shown to be plausible if the reverberation is keyed to the onset of neurally initiated visual disruptions such as blinks and saccades. Neural mechanisms suppress perception during these events but little is known about temporal filling-in processes analogous to the mechanisms that fill-in spatial scotomas. Crick's reverberatory loop could provide a process for filling-in temporal scotomas with information acquired just before the disruption, thus maintaining the continuity of visual experience.

Evidence of spatial and temporal channels in the correlational structure of human spatiotemporal contrast sensitivity.

1. The statistical correlation of detection thresholds for pairs of stimuli should be higher for stimuli detected by the same mechanism than for stimuli detected by different mechanisms--a property that can be used to probe the visual mechanisms that underlie detection. 2. Correlation of contrast sensitivities for pairs of spatiotemporal stimuli is approximately a linear function of spatial or temporal frequency separation in octaves. Using the slope of this function as an index of neural processing gave results consistent with: more spatial mechanisms than temporal; more spatial mechanisms at low temporal frequencies than at high; and at least two temporal mechanisms active at spatial frequencies up to 22.6 cycles deg-1. 3. This method of analysing sensitivity data is insensitive to experimental conditions and applicable to any sensory detection task mediated by tuned channels. In addition to being applicable to psychophysical sensitivity measurements, it may also be useful in analysing some kinds of electrophysiological measurements that pool the responses from many active mechanisms (such as evoked potentials).

Cortical simple cells can extract achromatic information from the multiplexed chromatic and achromatic signals in the parvocellular pathway.

P cells, which carry both achromatic and chromatic information, are largely responsible for achromatic acuity and contrast sensitivity. The P cell achromatic information must be separated from the chromatic information to be useful. Cortical simple cells are well suited to the extraction of achromatic information by spatial bandpass filtering. Bandpass filtering of Type I P cells by cortical simple cells yields an achromatic signal with a residual chromatic response. The bandpass model makes predictions in accord with existing physiological data and explains the role of a heretofore puzzling class of cortical cells, which have bandpass tuning for both achromatic and chromatic modulations. The model is shown to be related to a previously postulated class of ideal detectors. Finally, the model is used to make a number of physiological and psychophysical predictions.

Red-green opponent channel mediation of control of human ocular accommodation.

1. It has been hypothesized, but not verified empirically, that the control of human ocular accommodation is mediated by either the red-green or yellow-blue colour channels. Our goal was to determine experimentally whether the red-green channel by itself could influence the accommodative response. 2. To find out, we isolated the red-green channel through chromatic bandpass filtering and measured accommodation under dynamic and static conditions. The effect of this filtering was to modulate the red-green channel without disturbing either the yellow-blue or luminance channels. 3. Accommodative gain (ratio of response to stimulus amplitude) declined monotonically with decreasing bandwidth under dynamic conditions. Because the outputs of both the luminance and yellow-blue colour channels did not vary with bandwidth, the only explanation is that the red-green opponent process was responsible for the effect. 4. Under static conditions, however, accommodation was independent of bandwidth. This may be attributable to the decreased sensitivity to chromatic contrast that occurs at low temporal frequencies.

Opponent-color detection threshold asymmetries may result from reduction of ganglion cell subpopulations.

Thresholds for psychophysically opposite stimuli--light and dark, or equiluminous red and green, or equiluminous blue and yellow--are usually nearly equal. This color threshold symmetry is sometimes violated in subjects who have optic nerve hypoplasia, a congenital loss of retinal ganglion cells. We describe a subject who has optic nerve hypoplasia, who exhibits large red-green and blue-yellow detection threshold asymmetries for equiluminous spots. Temporal and spatial integration for equiluminous red and green test spots also differed from normal; static perimetric thresholds for equiluminous green, blue, and yellow (but not red) spots lacked the normal "V" shaped minimum at the fovea. These asymmetries may relate to a developmental paucity of some ganglion cell subtypes. Optic nerve hypoplasia may allow the contributions to detection made by individual ganglion cell subtypes to be isolated psychophysically, in analogy to the study of cone spectral sensitivity in dichromats.

The relationship between simple and double opponent cells.

Little is known about the formation of double opponent cells (DOCs) from geniculate afferents. Three LGN cell types have been considered as DOC precursors. No simple wiring scheme based on these cell types is consistent with the available evidence. The color and luminance multiplexed signal of P beta ganglion cells (Type I receptive fields) contains the information necessary to construct DOCs, provided that filtering operations can separate the two signals. Electrophysiological and anatomical evidence is consistent with Type I cells being filtered prior to the formation of DOCs. Cortical Type II and Type III cells can also be created by filtering.

Phase channels and the square-wave illusion.

We have utilized a phase-sensitive effect, the square-wave illusion, to investigate the presence and tuning of phase-sensitive mechanisms. Subjects viewed a 1 c/deg triangular-wave grating and reported the presence or absence of the illusion, before and after adaptation to square-wave gratings of various spatial frequencies. Illusion strength declined with adaptation and was a function of spatial frequency. This function may reflect the spatial frequency tuning of a 'phase channel'. We have also measured by adaptation a contrast sensitivity function, maximally sensitive to a frequency of 1 c/deg. When normalized to the same maximal depression from baseline sensitivity, the phase channel and spatial-frequency channel shared the same low frequency tuning, but the phase channel was broader, with extended sensitivity to higher frequencies. These results can be understood if the phase channel is constructed from combinations of phase sensitive spatial-frequency channels.