Properties of area 17/18 border neurons contributing to the visual transcallosal pathway in the cat

Abstract

In a series of physiological experiments, a total of 203 neurons at the Area 17/18 border were recorded with a callosal link either demonstrated by antidromic or transsynaptic activation from stimulating electrodes located in the homotopic contralateral hemisphere (CH), or in the splenial segment of the corpus callosum (CC). Forty-four percent of the transcallosal cells could also be driven from stimulating electrodes in or just above the lateral geniculate nucleus (OR1). The majority (69%) of transcallosal neurons were classifiable as belonging to the complex family (B and C cells) and most of these were found in the supragranular laminae and in lamina 4A. The ocular dominance distribution of transcallosal cells was trimodal, consisting of roughly equal numbers of monocularly dominated and binocularly balanced neurons. Estimates of conduction time and synaptic delay were obtained for neurons driven from CH, CC, and from OR1, and in most instances the response latency was short enough to suggest a monosynaptic input from either the ipsi- or contra-lateral hemisphere. The distribution of transcallosal conduction times showed that S cells, as a class, had significantly faster conduction than cells of the complex family but otherwise there was no obvious signs of multimodality in the distribution curve. An analysis of the synaptic delays in transcallosal activation produced a mean of 0.6 to 0.7 ms but some were too short to be consistent with a transsynaptic drive, suggesting that some cells with an antidromic drive may have been included in the transsynaptic category. Results are interpreted in terms of the contribution made by the corpus callosum to stereoscopic vision.

Scene segmentation by spike synchronization in reciprocally connected visual areas. II. Global assemblies and synchronization on larger space and time scales

“…The standard explanation is to assume that the delays are all sufficiently small, i.e., smaller than one-quarter or one-third of the oscillation period (e.g., Sompolinsky et al 1990;Ritz et al 1994). Since between visual areas in the same as well as in different hemispheres fast oscillatory synchronization at > 50 Hz have been observed (e.g., Eckhorn et al 1988;Singer and Gray 1995), this would restrict the respective inter-areal delays to values below 5-7 ms in accordance with some experimental results (Harvey 1980;Innocenti 1980;McCourt et al 1990). But contradictory evidence exists at least for the interhemispheric connections via the corpus callosum: a large fraction of the axons have been found to have transmission times in the range of the tens of milliseconds, and the measured values may even be a strong underestimation since slow-conducting axons are difficult to detect (Swadlow 2000).…”

Callosal connections correlate preferentially with ipsilateral cortical domains in cat areas 17 and 18, and with contralateral domains in the 17/18 transition zone

“…In this case, synchronous firing in both hemispheres would be elicited in territories dominated by the contralateral eye. Assuming that input from each eye preferentially activates callosal cells and targets of callosal axons that are located in spatial register with the territory dominated by the same eye (Toyama et al, 1974;McCourt et al, 1990), the biocular mechanism in Figure 9A predicts that callosal cells associated with contralateral ODCs will maintain their callosal axons, whereas callosal cells overlying ipsilateral ODCs will tend to lose their callosal axons. This process would lead to the segregation of callosal clusters that are preferentially correlated with contralateral ODCs both in the TZ as well as in regions located outside the TZ (see bottom of Fig.…”

Inhibition synchronizes sparsely connected cortical neurons within and between columns in realistic network models

“…Time delays between cells in the same column, separated by tens of pm, are l-2 ms due to conduction along thin, unmyelinated axons (Mason et al, 1991;Thomson et al, 1988). Time delays between cells in opposite hemispheres, separated by several cm, have been recorded at or below 5 ms due to conduction along thick, myelinated axons (McCourt et al, 1990). It would appear that the axons are organized to put cortical cells functionally next to each other.…”