Responses of Neurons in the Middle Temporal Visual Area After Long-Standing Lesions of the Primary Visual Cortex in Adult New World Monkeys

Abstract

The retinotopic organization of the middle temporal visual area (MT) was determined in six adult owl monkeys and one adult marmoset 69 d to 10 months after lesions of the dorsolateral primary visual cortex (V1). The lesions removed were limited to extensive parts of the representation of the lower visual quadrant in V1. Microelectrodes were used to record from neurons at numerous sites in MT to determine whether parts of MT normally devoted to the lower visual quadrant (1) were unresponsive to visual stimuli, (2) acquired responsiveness to inputs from intact portions of V1, or (3) became responsive to some other visually driven input such as a relay from the superior colliculus via the pulvinar to MT. All monkeys (n ϭ 6) with moderate to moderately large lesions had unresponsive portions of MT even after 10 months of recovery. These unresponsive regions were retinotopically equivalent to the removed parts of V1 in normal animals. Thus, there was no evidence for an alternative source of activation. In addition, these results indicate that any retinotopic reorganization of MT based on inputs from intact portions of V1 was not extensive, yet neurons near the margins of responsive cortex may have acquired new receptive fields, and the smallest 5°lesion of V1 failed to produce an unresponsive zone. Deprived portions of MT were not remarkably changed in histological appearance in cytochrome oxidase, Nissl, and Wisteria floribunda agglutinin preparations. Nevertheless, some reduction in myelin staining and other histological changes were suggested. We conclude that MT is highly dependent on V1 for activation in these monkeys, and alternative sources do not become effective over months when normal activation is absent. Additionally, remaining V1 inputs have only a limited capacity to expand their activation territory into deprived portions of MT.

Morphological comparison of visual pathway projections to the temporal lobe from cortical area VI and the tectorecipient zone of the pulvinar nucleus in the tree shrew (Tupaia belangeri).

“…Our TP projection pattern correlates to findings by Lyon et al 2003. That research found that PD projects to TP, but PC does not.…”

“…From those findings and ours, further research would need to be done to determine if the far ventral portion of PC does project to TA, or if the projections we are seeing are from PP, even though our micrographs of the injection site do not uncover tracer from that injection as far ventral as PP. Also in that research by Lyon et al (2003), they found that PP and not PC projected to TI. This projection in our study is most likely a PP to TI projection.…”

“…Projections from PC (Lyon et al 2003;Luppino et al 1988) and PD (Luppino et al 1988) to V2 have been previously reported. These two studies used retrograde tracer injections in V2 to identify cell bodies in the thalamus that project to V2.…”

“…The PP division of the tree shrew pulvinar was proposed by Lyon et al (2003) because of distinctly different connection patterns at the posterior pole of PC than in the rest of Pc. These unusual connections were rostral to the connections made by PC as a whole, such as the temporal inferior area (TI).…”

“…None of our anterograde injections in the pulvinar nucleus regions PC and PD projected to V2. In reviewing those studies, the lack of PD to V2 projections in the more recent study by Lyon et al (2003) may indicate that the injections by Luppino et al (1988) overran the boundary of V2 to TP (Luppino et al 1998 figure 8) and V2 to TD (Luppino et al 1998 figure 9), so that the cell bodies found in PD came from TP and I or caudal TD.…”