Timing of pure tone and noise-evoked responses in macaque auditory cortex.

We compared onset latencies for characteristic frequency pure tone and broadband noise responses in AI and posterior belt regions of the auditory cortex in awake macaques. We found that (1) in AI, responses to characteristic frequency tones and broadband noise have similar latencies, (2) in belt regions, characteristic frequency tone and broadband noise latencies differ significantly; broadband noise latencies are shorter, while characteristic frequency tone latencies are longer than corresponding values in AI, (3) for both pure tone and broadband noise responses in AI, latency decreases with increasing characteristic frequency and (4) despite a similar inverse relationship of tone latency and local characteristic frequency in belt areas, broadband noise latencies are uniformly short, and appear unrelated to local characteristic frequency. Dissociation of broadband noise and pure tone latencies may reflect the use of parallel anatomical routes into belt regions.

Attention and arousal related modulation of spontaneous gamma-activity in the auditory cortex of the cat.

Sensory information processing in neocortex is associated with rhythmic synchronized gamma frequency firing of sensory cortical units and similar frequency oscillations of the field potentials. Different aspects of the gamma activity (20-80 Hz) have been suggested as correlates of attention, arousal and sensory binding. It is clear that attention has a modality selective influence, while arousal has a more general effect on the sensory systems. We used an experimental conditioning paradigm to separate these differential effects of attention and arousal on spontaneous neocortical gamma activity. We recorded field potentials with epidural electrodes placed above the auditory cortical areas of cats. The animals performed a simple instrumental alimentary conditioning task with different modality (visual and auditory) conditioned stimuli. When they attended to the auditory conditioned stimulus, both frequency and power increase of spontaneous gamma activity were detected. However when they attended visual, we found no power increase of gamma activity recorded above auditory areas, while the frequency increase was the same as in the "attend auditory" condition. We conclude that the power modulation of gamma activity is modality specific and thus can be attributed to selective attention, whereas the frequency modulation of gamma activity shows no modality specificity, it is influenced by the arousal level.

Frequency of gamma activity is modulated by motivation in the auditory cortex of cat.

Repetitive acoustic stimuli elicit steady-state response (SSR) in the gamma-band both in humans and in mammals. Our aim was to investigate changes of the spontaneous gamma activity and the SSR in the auditory cortex of cats in the background of an instrumental conditioning situation. Epidural electrodes were chronically implanted above the auditory neocortex. The presentation rate of the clicks varied between 20 and 65/s. Spontaneous EEG and SSR were collected in three behavioral states: in an indifferent environment, in the instrumental cage while the cat was waiting for the light CS, and when she stepped on the pedal and was waiting for the meat reward. Using different repetition rate clicks we determined which stimulus rate elicited the largest SSR in these three situations. In quiet animal the highest SSR appeared at 28-30/s. Before and during the CS the optimal stimulus rate shifted to 32-38/s. The frequency of the spontaneous gamma activity changed in parallel way depending on the situation. We conclude that both the SSR and the spontaneous gamma activity reflect resonant activity of the same neuronal circuit of the auditory cortex, and it is modulated by the motivational state of the animal.

Effect of deviant probability and interstimulus/interdeviant interval on the auditory N1 and mismatch negativity in the cat auditory cortex.

In passive oddball paradigm the effects of changes in interstimulus/interdeviant interval (ISI; IDI) and deviant probability were investigated on mismatch negativity (MMN), auditory N1 wave and the exogenous P1 component of the auditory event-related potential in the cat. An epidural electrode matrix was chronically implanted over the auditory fields of the neocortex, and the amplitudes of the aforementioned components were measured in the location of their amplitude maxima. Dependence of the MMN both on the ISI and IDI as well as deviant probability was revealed, while the amplitude of the P1 and N1 showed dependence merely on the ISI. This method can be used for separation of the two negative, often overlapping components in the cat.