Auditory evoked potentials to spectro-temporal modulation of complex tonesin normal subjects and patients with severe brain injury

In order to assess higher auditory processing capabilities, long-latency au ditory evoked potentials (AEPs) were recorded to synthesized musical instru ment tones in 22 post-comatose patients with severe brain injury causing va riably attenuated behavioural responsiveness. On the basis of normative stu dies, three different types of spectre-temporal modulation were employed. W hen a continuous 'clarinet) tone changes pitch once every few seconds, N1/P 2 potentials are evoked at latencies of similar to 90 and 180 ms, respectiv ely. Their distribution in the fronto-central region is consistent with gen erators in the supratemporal cortex of both hemispheres. When the pitch is modulated at a much faster rate (similar to 16 changes/s), responses to eac h change are virtually abolished but potentials with similar distribution a re still elicited by changing the timbre (e.g. 'clarinet' to 'oboe') every few seconds. These responses appear to represent the cortical processes con cerned with spectral pattern analysis and the grouping of frequency compone nts to form sound 'objects'. Following a period of 16/s oscillation between two pitches, a more anteriorly distributed negativity is evoked on resumpt ion of a steady pitch. Various lines of evidence suggest that this is proba bly equivalent to the 'mismatch negativity' (MMN), reflecting a pre-percept ual, memory-based process for detection of change in spectrotemporal sound patterns. This method requires no offline subtraction of AEPs evoked by the onset of a tone, and the MMN is produced rapidly and robustly with conside rably larger amplitude (usually >5 mu V) than that to discontinuous pure to nes. In the brain-injured patients, the presence of AEPs to two or more com plex tone stimuli (in the combined assessment of two authors who mere 'blin d! to the clinical and behavioural data) was significantly associated with the demonstrable possession of discriminative hearing (the ability to respo nd differentially to verbal commands, in the assessment of a further author who was blind to the AEP findings). Behavioural and electrophysiological f indings were in accordance in 18/22 patients, but no AEPs could be recorded in two patients who had clear behavioural evidence of discriminative heari ng. The absence of long-latency AEPs should not, therefore, be considered i ndicative of complete functional deafness. Conversely, AEPs mere substantia lly preserved in two patients without behavioural evidence of discriminativ e hearing. Although not necessarily indicative of conscious 'awareness', su ch AEP preservation might help to identify sentient patients who are preven ted by severe motor disability from communicating their perception.