Abstract
One leading hypothesis on the nature of visual callosal connections (CC) is that they replicate features of intrahemispheric lateral connections. However, CC act also in the central part of the binocular visual field. In agreement, early experiments in cats indicated that they provide the ipsilateral eye part of binocular receptive fields (RFs) at the vertical midline (Berlucchi and Rizzolatti, 1968), and play a key role in stereoscopic function. But until today callosal inputs to receptive fields activated by one or both eyes were never compared simultaneously, because callosal function has been often studied by cutting or lesioning either corpus callosum or optic chiasm not allowing such a comparison. To investigate the functional contribution of CC in the intact cat visual system we recorded both monocular and binocular neuronal spiking responses and receptive fields in the 17/18 transition zone during reversible deactivation of the contralateral hemisphere. Unexpectedly from many of the previous reports, we observe no change in ocular dominance during CC deactivation. Throughout the transition zone, a majority of RFs shrink, but several also increase in size. RFs are significantly more affected for ipsi- as opposed to contralateral stimulation, but changes are also observed with binocular stimulation. Noteworthy, RF shrinkages are tiny and not correlated to the profound decreases of monocular and binocular firing rates. They depend more on orientation and direction preference than on eccentricity or ocular dominance of the receiving neuron's RF. Our findings confirm that in binocularly viewing mammals, binocular RFs near the midline are constructed via the direct geniculo-cortical pathway. They also support the idea that input from the two eyes complement each other through CC: Rather than linking parts of RFs separated by the vertical meridian, CC convey a modulatory influence, reflecting the feature selectivity of lateral circuits, with a strong cardinal bias.
Highlights
It has been proposed that visual callosal connections (CC) perpetuate the function of intrahemispheric lateral connections across the two visual hemifields
In order to investigate the functional contribution of callosal connections to monocularly and binocularly activated receptive fields and binocular mechanisms in an intact system we studied spiking responses in the receiving 17/18 transition zone (TZ) during reversible deactivation of the sending hemisphere in cats
For the analysis of changes in ocular dominance and receptive field size, we considered 141 single units from 5 cats with receptive fields (RFs) for both monocular and binocular stimulation during baseline, cooling and recovery, 76 from matrix electrodes placed in the TZ, and, for comparison, 42 from electrodes within the area 17 adjacent to the TZ and 23 from electrodes placed into area 18 adjacent to the TZ
Summary
It has been proposed that visual callosal connections (CC) perpetuate the function of intrahemispheric lateral connections across the two visual hemifields. Experiments sectioning the corpus callosum or lesioning the contralateral cortex claimed that CC contribute a major part to the binocularity of callosal neurons in cats (striate: Dreher and Cottee, 1975; Payne et al, 1980, 1984; Blakemore et al, 1983; Yinon et al, 1988; extrastriate: Marzi et al, 1980) This result was not confirmed by other studies (Zeki and Fries, 1980; Lepore et al, 1983; Minciacchi and Antonini, 1984; Gardner and Cynader, 1987). In order to investigate the functional contribution of callosal connections to monocularly and binocularly activated receptive fields and binocular mechanisms in an intact system we studied spiking responses in the receiving 17/18 transition zone (TZ) during reversible deactivation of the sending hemisphere in cats
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