Thalamic mechanisms in language: a reconsideration based on recent findings and concepts

Abstract

Recent literature on thalamic aphasia and thalamic activity during neuroimaging is selectively reviewed followed by a consideration of recent anatomic and physiological findings regarding thalamic structure and functions. It is concluded that four related corticothalamic and/or thalamocortical mechanisms impact language processing: (1) selective engagement of task-relevant cortical areas in a heightened state of responsiveness in part through the nucleus reticularis (NR), (2) passing information from one cortical area to another through corticothalamo-cortical mechanisms, (3) sharpening the focus on task-relevant information through corticothalamo-cortical feedback mechanisms, and (4) selection of one language unit over another in the expression of a concept, accomplished in concert with basal ganglia loops. The relationship and interaction of these mechanisms is discussed and integrated with thalamic aphasia and neuroimaging data into a theory of thalamic functions in language.

Published by Elsevier Inc.

Figures

Figure 1

Schematic drawing of the fronto-inferior thalamic peduncle-nucleus reticularis- centromedian system that Nadeau & Crosson (1997) hypothesized to be responsible for selective engagement of cortical nets necessary for processing semantic properties of specific stimuli. CM = centromedian nucleus of the thalamus, IML = internal medullary lamina, ITP = inferior thalamic peduncle, MRF = midbrain reticular formation, NR = nucleus reticularis, NRva = ventral anterior nucleus reticularis, VA = ventral anterior nucleus of the thalamus. From S.E. Nadeau & B. Crosson. (1997). Subcortical aphasia. Brain & Language 58(3), 355–402, reprinted by permission of Elsevier.

Figure 2

This schematic represents the experiment of Theyel et al. (2010). They cut the cortico-cortical connections between S1 and S2 in the mouse brain in vitro. Under these conditions, S2 could still be activated by stimulating S1. When the thalamic component was chemically inactivated, it was no longer possible to evoke a response in S2 by stimulating S1. After washout of the chemical inactivation, the ability to activate S2 by stimulating S1 was reestablished, demonstrating that it was the corticothalamo-cortical connection that was responsible for activating S2 during S1 stimulation. L5 = cortical layer 5, POm = posteromedial nucleus of the thalamus, S1 = primary somatosensory cortex, S2 = secondary somatosensory nucleus.

Figure 3

Sillito et al (2006) discussed how in the visual system, area MT can influence primary visual cortex through corticothalamic feedback mechanisms. MT projects to V1, including layer 6, where contact can be made with corticothalamic cells. The modulatory input from V1 layer 6 to the lateral geniculate nucleus in can influence the thalamocortical relay cell, allowing MT to sharpen features in M1 needed for MT to resolve movement-related information. L6 = cortical layer 6, LGN = lateral geniculate nucleus, MT = middle temporal cortex involved in visual perception of movement, NR = nucleus reticularis, V1 = primary visual cortex.

Figure 4

This figure is a conceptual representation of how basal ganglia loops may influence word production during a semantic fluency task. In this example, the subject is asked to produce as many birds as possible. During the previous trial (time −1), “eagle” was produced while competing birds were suppressed (white background indicates an activated concept, while gray represents suppressed concepts, with darker gray indicating greater suppression than lighter gray). When it comes time to generate a new bird (time 0), “eagle” is suppressed by the hyperdirect pathway so that “crow” can take its place. At time +1, “crow” is activated by the direct pathway, but competing responses are also somewhat activated. At time +2, competing concepts are suppressed, increasing the signal-to-noise ratio, and minimizing the probability of errors in production. From B. Crosson, M. Benjamin, & I. Levy. (2007) Role of the basal ganglia in language: Supporting Cast. In J. Hart & M. Kraut (eds.), Neural Basis of Semantic Memory, (pp. 219–233. New York: Cambridge University Press. Reprinted by permission of Cambridge University Press.

Figure 5

Four thalamic mechanisms proposed to affect language. (a) Selective Engagement. This schematic represents a revised selective engagement mechanism whereby anterior Broca’s area (BAa) contacts the nucleuse reticularis (NR). NR in turn places parts of the pulvinar necessary to represent features (e.g., visual form and motor sequences) of a hammer in a state of selective engagement. (b) Transfer of Information. This schematic represents passing information about the features of hammer on to a pattern associator in the inferior parietal lobe that takes the features as an input and produces the correct word as an output. (c) Sharpening the Focus. The observed motion involved in swinging a hammer can be sharpened by feedback from MR to V1 layer 6 (L6), which in turn, influences thalamocortical relays in the lateral geniculate nucleus (LGN). Not shown is that LGN relay cells, in turn, can influence V1 information necessary to identify the movement. (d) Lexical Selection. This schematic represents the hyperdirect, direct, and indirect loops for pre-SMA. Also shown is the possibility that layer 6 corticothalamo-cortical feedback might influence the output of these loops from the ventral anterior nucleus to the thalamus. BAa = anterior Broca’s area, GPi = internal globus pallidus, GPe = external globus pallidus, CNd = dorsal caudate nucleus, FG = fusiform gyrus, iml = internal medullary lamina, IPL = inferior parietal lobule, L6 = cortical layer 6, LGN = lateral geniculate nucleus, NR nucleus reticularis, Pl = pulvinar, PPC = posterior perisylvian cortex, STN = subthalamic nucleus, V1 = primary visual cortex, VA = ventral anterior nucleus.