Tinnitus by Jos J. Eggermont Fan-Gang Zeng Arthur N. Popper & Richard R. Fay

Author:Jos J. Eggermont, Fan-Gang Zeng, Arthur N. Popper & Richard R. Fay
Language: eng
Format: epub
Publisher: Springer New York, New York, NY

7.2 Cortical Influences on Subcortical Structures

Yan and Suga (1998) found in the big brown bat that electrical stimulation of a column in primary auditory cortex paired with a tone of frequency equal to the best frequency (BF) of the cortical column, enhances the extent of the corresponding frequency representation in the IC. Moreover, the plastic changes were asymmetric; IC neurons with best frequencies higher than that of the stimulated cortical site showed downward shifts in their best frequencies, toward that of the stimulated neurons, whereas neurons tuned to lower frequencies were relatively unaffected. Surprisingly, 30 minutes of stimulation with tone bursts presented at 50 dB SPL without concurrent electrical stimulation also induced a shift in the frequency tuning in the IC. The changes were smaller than but similar to those observed after combined tone and electrical stimulation of the corresponding BF site in the auditory cortex. Thus, behaviorally irrelevant tone bursts and/or direct cortical electrical stimulation can augment the midbrain representation of the stimulus tone frequency.

This effect was also demonstrated in mice (Yan & Ehret, 2001, 2002; Yan et al., 2005). Yan and colleagues observed that bipolar electrical stimulation of primary auditory cortex, one electrode at the surface, the other one in layer VI, so spanning the depth of a cortical column, did not affect best frequencies in IC when the BFs of stimulated cortical neurons and recorded collicular neurons were similar. However, BFs in IC shifted toward the BF of the cortical stimulation site when cortical and collicular frequencies were different. In addition to frequency-specific shifts in collicular BFs, cortical stimulation elevated minimum thresholds in IC and reduced both dynamic ranges and response magnitudes if cortical and collicular BFs were different. If BFs in AI and IC were similar but minimum thresholds were different, collicular minimum thresholds shifted toward the thresholds of the stimulated cortical sites.

He et al. (2002) found similar modulatory effects descending from the auditory cortex to the thalamus in guinea pigs. Corticofugal modulation on thalamic neurons was again obtained by electrical activation of auditory cortex. Neuronal activity was recorded along the frontal and sagittal planes of the auditory thalamus, focusing on the ventral division (MGv) of the medial geniculate body (MGB). The corticofugal effect on the MGv of the guinea pig resulted in strong facilitation and very little inhibition. The MGv neurons showed the greatest facilitations to stimulation of cortical sites with the closest correspondence in BF. The comparative results of the corticofugal modulatory effects on the MGv of the guinea pig and the cat, together with anatomical findings, hint that the strong facilitatory effect is generated through the strong direct corticothalamic connection to thalamic principal cells and that the weak inhibitory effect might be mainly generated via the interneurons of the MGv (He, 1997).

In exploring the corticofugal effects on levels below the midbrain, Luo et al. (2008) found that cortical activation increased the response magnitudes and shortened response latencies of CN neurons with BFs matched to the cortical stimulation site, whereas decreased response magnitudes and lengthened response latencies of unmatched CN neurons.

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