Neurons in sensory cortices are targeted by cholinergic projections from the basal forebrain that modulate synaptic plasticity by making neurons more sensitive to their afferent inputs. This suggests that the basal forebrain system, which is corticotopically organised, may perform some of the functions of an attention mechanism. Consistent with this hypothesis, we have shown that the amplitude of the 40-Hz auditory steady state response (aSSR), which localises to A1 and demonstrates a tonotopic ordering, is enhanced by attention. aSSR amplitude increased when subjects detected targets in a 1-s stream of 40-Hz stimulation compared to conditions in which subjects passively experienced the stimuli. In an experiment designed to test frequency specific modulation of primary auditory cortex using two simultaneously presented carrier frequencies our findings suggested that attention cannot selectively activate specific tonotopic regions in A1.

Hebb Award Abstract
The present study investigated whether attentional modulation of early cortical regions by top-down attention can be modality specific. Subjects (n=16) were asked to attend to one of two stimulus modalities (auditory or visual) presented simultaneously in 1-sec trials and amplitude modulated at different rates (AM, 16 Hz visual, or 41 Hz auditory) to elicit a distinct steady-state brain response in each modality (auditory: aSSR, visual: vSSR). The auditory and visual stimuli were presented at matched, moderate intensity with targets embedded in each. Targets were AM pulses of enhanced amplitude that occurred independently in visual and auditory stimuli randomly throughout the latter half of the 1-sec stimulus, requiring sustained attention throughout a trial. Following ten minutes of passive stimulation, ten active blocks of 2-min duration were delivered in blocks separated by a brief rest. In the first half of the attention task participants were asked to press a button to auditory and, in the second half, to visual targets (order counterbalanced). Following behavioural performance a final ten minutes of passive stimulation was presented. Savings were assessed in subjects that returned for a second session between 24 and 72 hours. Behavioural performance in each modality was assessed across five target intensities surrounding the subject’s threshold. aSSR and vSSR were assessed by DFT applied to the 128-channel EEG average.
Behavioural analyses showed orderly psychophysical functions for both the auditory and visual stimulus. Subjects therefore complied with the attentional requirements of the task. Distinctive 16 Hz vSSR and 41 Hz aSSRs were generated by each modulated stimulus. Modulation of the brain response from the first passive stage was found for both the aSSR and vSSR under active attention, and returned to baseline levels during the second passive condition. A selective attention effect was found for both vision and audition as a larger brain response when attended than when unattended, with unattended response amplitude remaining the same as under passive stimulation. No effects of phase were observed across conditions. No evidence was found for learned brain changes on the second session. These results demonstrate that selective attention can modulate individual sensory modalities, and together with our earlier findings suggest rules governing the operation of attention in cortex.