Spatiotopic Representation Research Articles

Retinotopic and non-retinotopic stimulus encoding in binocular rivalry and the involvement of feedback

Adaptation is one of the key constituents of the perceptual alternation process during binocular rivalry, as it has been shown that preadapting one of the rivaling pairs before rivalry onset biases perception away from the adapted stimulus during rivalry. We investigated the influence of retinotopic and spatiotopic preadaptation on binocular rivalry. We show that for grating stimuli, preadaptation only influences rivalry when adaptation and rivalry locations are retinotopically matched. With more complex house and face stimuli, effects of preadaptation are found for both retinotopic and spatiotopic preadaptation, showing the importance of spatiotopic encoding in binocular rivalry. We show, furthermore, that adaptation to phase-scrambled faces results in retinotopic effects only, demonstrating the importance of form content for spatiotopic adaptation effects, as opposed to spatial frequency content. Are the spatiotopic adaptation influences on rivalry caused by direct spatiotopic stimulus interactions, or instead are they due to altered feedback from the adapted spatiotopic representations to the retinotopic representations that are involved in rivalry? By using rivaling face and grating stimuli that minimize rivalry between spatiotopic representations while still engaging these representations in stimulus encoding, we show that at least part of the preadaptation effects with face stimuli depend on feedback information.

Object localization in cluttered acoustical environments

In nature, sounds from objects of interest arrive at the ears accompanied by sound waves from other actively emitting objects and by reflections off of nearby surfaces. Despite the fact that all of these waveforms sum at the eardrums, humans with normal hearing effortlessly segregate one sound source from another. Our laboratory is investigating the neural basis of this perceptual feat, often called the "cocktail party effect", using the barn owl as an animal model. The barn owl, renowned for its ability to localize sounds and its spatiotopic representation of auditory space, is an established model for spatial hearing. Here, we briefly review the neural basis of sound-localization of a single sound source in an anechoic environment and then generalize the ideas developed therein to cases in which there are multiple, concomitant sound sources and acoustical reflection.

Spatiotopic selectivity of BOLD responses to visual motion in human area MT

Many neurons in the monkey visual extrastriate cortex have receptive fields that are affected by gaze direction. In humans, psychophysical studies suggest that motion signals may be encoded in a spatiotopic fashion. Here we use functional magnetic resonance imaging to study spatial selectivity in the human middle temporal cortex (area MT or V5), an area that is clearly implicated in motion perception. The results show that the response of MT is modulated by gaze direction, generating a spatial selectivity based on screen rather than retinal coordinates. This area could be the neurophysiological substrate of the spatiotopic representation of motion signals.

Dissociating Sensory and Motor Components of Inhibition of Return

Two explanations for inhibition of return (IOR) have been proposed. The first is that IOR reflects inhibition of attentional processing at previously cued locations, resulting in altered sensory analysis. The second is that IOR reflects the inhibition of responses directed towards those previously cued locations. We used a variant of a double-saccade paradigm to dissociate these two proposed effects of IOR and attempted to reveal both effects within the context of a single experimental task. Subjects viewed a series of exogenous cues and then made a localization response to subsequent targets with either a target-directed saccade or a pointing response. Results were similar for both response modes. An important finding was that the pattern of IOR depended critically on how subjects reacted to the exogenous cues. Subjects either oriented to the cued locations (via saccades or pointing) prior to responding to the target (Respond), or passively viewed the cues before responding (Ignore). In the Respond condition, IOR was observed at the most recently cued position. Although this could be consistent with an altered sensory interpretation, it would also be consistent with a spatiotopic representation. In the Ignore condition, the sole inhibited location was not the most recently cued position, but the first cued position. This finding is surprising and in conflict with previous work with multiple exogenous cues. The data are discussed in relation to a number of prominent issues in the area of IOR and suggest important new constraints and boundary conditions.

The existence and role of retinotopic and spatiotopic forms of visual persistence

Current theoretical conceptualizations of visual persistence fail to address the issue of its functional role. Based on extant experimental data we tentatively identify two general types of visual persistence: one resides in activity along the afferent visual pathway and is retinotopically organized; the other resides at central levels and is spatiotopically organized. Moreover, whereas the former afferent persistence is eliminated via saccadic suppression mechanisms in order to separate successive, retinotopic frames of pattern information, the latter, central one, in contrast, is generated and enhanced via extraretinal signals accompanying saccades in order to preserve phenomenal continuity of a stable spatiotopic representation of the environment from one fixation to the next.