Intrathalamic mechanisms of visual attention

Abstract

The classical model of visual processing emphasizes the lateral geniculate nucleus (LGN) as the major intermediary between the retina and visual cortex. Yet, anatomical findings inspired Francis Crick to suggest an alternative model in which the thalamic reticular nucleus, which envelops the LGN, acts as the "guardian" of visual cortex by modulating LGN activity. Recent work by McAlonan and colleagues supports Crick's hypothesis, thereby enhancing our understanding of the early stages of visual processing.

FIG. 1.

Simplified illustration of visual pathway from retina to visual cortex. A: visual information enters the retina and passes through visual thalamus (gray box), including LGN and TRN (italics). Excitatory (open arrow tails) and inhibitory (closed circle) connections are shown for retinal output and the LGN–TRN–V1 circuit. Other nuclei (circles) also likely influence our global visual percept. Note that only Pul and LGN are likely recipients of direct retinal input, as shown, and not all visual nuclei project directly to V1. LGN, lateral geniculate nucleus; MD, mediodorsal nucleus; Pul, pulvinar; TRN, thalamic reticular nucleus; V1, primary visual cortex; VL, ventrolateral nucleus; X, area X. B: key physiological results from McAlonan et al. (2008). Top left: in the ATTin condition, the focus of attention (gray disc) is aligned with the receptive field (dashed circle). Bottom left: in the ATTout condition, attention is fixed on the stimulus outside the receptive field. Right: schematic representation of neuronal responses in LGN and TRN. LGN neurons showed greater activity when attention was focused at the receptive field location (ATTin; black). In contrast, TRN neurons showed relatively larger responses when the focus of attention was outside the receptive field (ATTout; gray).