Biomedical subjects

Izumi Ohzawa

Publications and source records attributed to Izumi Ohzawa.

5 recordsLinked to original sources

PubMed2006

Neural basis for stereopsis from second-order contrast cues.

Humans and animals use visual cues such as brightness and color boundaries to identify objects and navigate through environments. However, even when these cues are not available, we can effortlessly perform these tasks by using second-order cues such as contrast variation (envelope) of patterns on surfaces. Previously, numerous psychophysical studies examined properties of binocular depth processing based on the contrast-envelope cues and suggested the existence of a stereo system that uses these cues. However, its physiological substrate has not been identified yet. Here, we show that a subset of cortical neurons in cat area 18 show binocular interactions for the contrast-envelope stimuli. These neurons are capable of representing a variety of depths in the three-dimensional space based on the information available from contrast cues alone. Furthermore, these neurons show similar disparity-tuning curves for borders defined by both luminance and contrast cues. This cue-invariant tuning is consistent with a linear binocular convergence model for monocular luminance and contrast-envelope processing pathways.

Animals↗

PubMed2006

Receptive field properties of neurons in the early visual cortex revealed by local spectral reverse correlation.

We introduce a novel class of white-noise analyses, named local spectral reverse correlation (LSRC), which is capable of revealing various aspects of visual receptive field profiles that were undetectable previously in a single simple measurement. The method is based on spectral analyses in a two-dimensional spatial frequency domain for spatially localized areas within and around their receptive fields. Extracellular single-unit recordings were performed for area 17 and 18 neurons in anesthetized cats. A dynamic dense noise pattern was presented in which the pattern covered an area two to three times larger than the classical receptive field. Spike trains were then cross-correlated with frequency spectra of localized noise pattern to obtain spatially localized selectivity maps in the two-dimensional frequency domain. Our findings are as follows. (1) The new LSRC method allows measurements of two-dimensional frequency tunings and their spatial extent even for cells with substantial nonlinearity. (2) A small subset of neurons shows spatial inhomogeneity in the two-dimensional frequency tunings. (3) In addition to facilitatory response profiles, we can also visualize suppressive profiles localized both in space and spatial frequency domains. Our results suggest that the new analysis technique can be a powerful tool for measuring visual response profiles that contain inhomogeneity in space, as well as for studying neurons with substantial nonlinearities. These features make the method particularly suitable for studying response profiles of neurons in early as well as intermediate extrastriate visual areas.

Action Potentials↗

PubMed2006

Encoding of three-dimensional surface slant in cat visual areas 17 and 18.

How are surface orientations of three-dimensional objects and scenes represented in the visual system? We have examined an idea that these surface orientations are encoded by neurons with a variety of tilts in their binocular receptive field (RF) structure. To examine whether neurons in the early visual areas are capable of encoding surface orientations, we have recorded from single neurons extracellularly in areas 17 and 18 of the cat using standard electrophysiological methods. Binocular RF structures are obtained using a binocular version of the reverse correlation technique. About 30% of binocularly responsive neurons have RFs with statistically significant tilts from the frontoparallel plane. The degree of tilts is sufficient for representing the range of surface slants found in typical visual environments. For a subset of neurons having significant RF tilts, the degrees of tilt are correlated with the preferred spatial frequency difference between the two eyes, indicating that a modified disparity energy model can account for the selectivity, at least partially. However, not all cases could be explained by this model, suggesting that multiple mechanisms may be responsible. Therefore an alternative hypothesis is also examined, where the tilt is generated by pooling of multiple disparity detectors whose preferred disparities progressively shift over space. Although there is evidence for extensive spatial pooling, this hypothesis was not satisfactory either, in that the neurons with extensive pooling tended to prefer an untilted surface. Our results suggest that encoding of surface orientations may begin with the binocular neurons in the early visual cortex.

Animals↗

PubMed2005

Accuracy of subspace mapping of spatiotemporal frequency domain visual receptive fields.

Orientation and spatial frequency selectivities are fundamental properties of cells in the early visual cortex. Although they are customarily tested with drifting sinusoidal gratings, a recently developed subspace reverse correlation method may be a better replacement for obtaining a selectivity map in a joint orientation and spatial frequency domain at higher resolution efficiently. These two methods are examined for their accuracy and data compatibility for cells in areas 17 and 18 of anesthetized and paralyzed cats. Peaks and bandwidths of tuning curves from these two methods are highly correlated. However, spatial frequency bandwidths obtained by reverse correlation tend to be slightly narrower for the subspace reverse correlation than those from the drifting grating tests. Consistency between the two methods is improved if the entire duration of data containing signal are taken into account for the subspace reverse correlation rather than using the map only at the optimal correlation delay. Examination of convergence of the subspace mapping process shows that reliable 2-day profiles can be obtained within 5-10 min. for the majority of cells. Temporal dynamics of tuning properties are also examined more directly with the subspace mapping than with the drifting gratings. For many cells, the optimal spatial frequency shifts substantially, measured as a fraction of tuning bandwidth, over the time course of response. In comparison, the optimal orientation remains highly stable throughout the duration of response. Overall, these results suggest that the subspace reverse correlation is a better substitute for the conventional method.

Action Potentials↗

PubMed2002

Disinhibition outside receptive fields in the visual cortex.

By definition, the region outside the classical receptive field (CRF) of a neuron in the visual cortex does not directly activate the cell. However, the response of a neuron can be influenced by stimulation of the surrounding area. In previous work, we showed that this influence is mainly suppressive and that it is generally limited to a local region outside the CRF. In the experiments reported here, we investigate the mechanisms of the suppressive effect. Our approach is to find the position of a grating patch that is most effective in suppressing the response of a cell. We then use a masking stimulus at different contrasts over the grating patch in an attempt to disinhibit the response. We find that suppressive effects may be partially or completely reversed by use of the masking stimulus. This disinhibition suggests that effects from outside the CRF may be local. Although they do not necessarily underlie the perceptual analysis of a figure-ground visual scene, they may provide a substrate for this process.

Animals↗