Biomedical subjects

Michael D Menz

Publications and source records attributed to Michael D Menz.

3 recordsLinked to original sources

PubMed2003

Temporal dynamics of binocular disparity processing in the central visual pathway.

To solve the stereo correspondence problem (i.e., find the matching features of a visual scene in both eyes), it is advantageous to combine information across spatial scales. The details of how this is accomplished are not clear. Psychophysical studies and mathematical models have suggested various types of interactions across spatial scale, including coarse to fine, fine to coarse, averaging, and population coding. In this study, we investigate dynamic changes in disparity tuning of simple and complex cells in the cat's striate cortex over a short time span. We find that disparity frequency increases and disparity ranges decrease while optimal disparity remains constant, and this conforms to a coarse-to-fine mechanism. We explore the origin of this mechanism by examining the frequency and size dynamics exhibited by binocular simple cells and neurons in the lateral geniculate nucleus (LGN). The results suggest a strong role for a feed-forward mechanism, which could originate in the retina. However, we find that the dynamic changes seen in the disparity range of simple cells cannot be predicted from their left and right eye monocular receptive field (RF) size changes. This discrepancy suggests the possibility of a dynamic nonlinearity or disparity specific feedback that alters tuning or a combination of both mechanisms.

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PubMed2003

Functional connectivity of disparity-tuned neurons in the visual cortex.

Different mechanisms have been proposed concerning how disparity-tuned neurons might be connected to produce the signals for depth perception. Here we present neurophysiological evidence providing insight on this issue. We have recorded simultaneously from pairs of disparity-tuned neurons in the cat's striate cortex. The purpose was to determine the relationships between disparity tuning and functional connectivity revealed through neural cross-correlograms. Monosynaptic connections tend to be stronger between pairs of cells with similar disparity tuning. Pairs of complex cells tend to have either similar tuning or nearly opposite tuning with an absence of quadrature relations. Pairs with at least one simple cell do have some nearly quadrature relationships when they are recorded from the same electrode. Coarse-to-fine connections (i.e., the presynaptic cell has lower disparity frequency and larger disparity range) tend to be stronger but less frequent than those of a fine-to-coarse nature. Our results are consistent with a system that produces weighted averaging across cells that are tuned to similar disparities but different disparity scales to reduce false matches.

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PubMed2003

Stereoscopic depth processing in the visual cortex: a coarse-to-fine mechanism.

For binocular animals viewing a three-dimensional scene, the left and right eyes receive slightly different information, and the brain uses this 'binocular disparity' to interpret stereoscopic depth. An important theoretical conjecture in this mechanism is that coarse processing precedes and constrains finely detailed processing. We present three types of neurophysiological data from the cat's visual cortex that are consistent with a temporal coarse-to-fine tuning of disparity information. First, the disparity tuning of cortical cells generally sharpened during the time course of response. Second, cells responsive to large and small spatial scale had relatively shorter and longer temporal latencies, respectively. Third, cross-correlation analysis between simultaneously recorded pairs of cortical cells showed that connections between disparity-tuned neurons were generally stronger for coarse-to-fine processing than for fine-to-coarse processing. These results are consistent with theoretical and behavioral studies and suggest that rapid, coarse percepts are refined over time in stereoscopic depth perception.

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