David L. Woods

Contributions of temporal-parietal junction to the human auditory P3

The P3 component of the event-related potential (ERP) is generated in humans and other mammalian species when attention is drawn to infrequent stimuli. We assessed the role of subregions of human posterior association cortex in auditory P3 generation in groups of patients with focal cortical lesions. Auditory P3s were recorded to target (P3b) and unexpected novel stimuli (P3a) in monaural and dichotic signal detection experiments. Two groups of patients were studied with lesions of: (1) temporal-parietal junction including posterior superior temporal plane and adjacent caudal inferior parietal cortex; and (2) the lateral parietal lobe including the rostral inferior parietal lobe and portions of superior parietal lobe. Extensive lateral parietal cortex lesions had no effect on the P3. In contrast, discrete unilateral lesions centered in the posterior superior temporal plane eliminated both the auditory P3b and P3a at electrodes over the posterior scalp. The results indicate that auditory association cortex in the human temporal-parietal junction is critical for auditory P3 generation.

Shape perception reduces activity in human primary visual cortex.

Visual perception involves the grouping of individual elements into coherent patterns that reduce the descriptive complexity of a visual scene. The physiological basis of this perceptual simplification remains poorly understood. We used functional MRI to measure activity in a higher object processing area, the lateral occipital complex, and in primary visual cortex in response to visual elements that were either grouped into objects or randomly arranged. We observed significant activity increases in the lateral occipital complex and concurrent reductions of activity in primary visual cortex when elements formed coherent shapes, suggesting that activity in early visual areas is reduced as a result of grouping processes performed in higher areas. These findings are consistent with predictive coding models of vision that postulate that inferences of high-level areas are subtracted from incoming sensory information in lower areas through cortical feedback.

Intermodal selective attention. II. Effects of attentional load on processing of auditory and visual stimuli in central space

The effect of processing load on event-related brain potentials (ERPs) was investigated in an intermodal selective attention task in which subjects attended selectively to auditory or visual stimuli. Processing load was manipulated by requiring subjects to detect either difficult-to-detect (deviant) or easy-to-detect (DEVIANT) targets in separate blocks of trials. Attention to auditory stimuli was associated with negative (Nda, 90-170 msec) and positive (Pda, 190-270 msec) enhancements in the ERPs to auditory stimuli. The Nda increased in amplitude with increasing processing load. Deviant auditory stimuli occurring among auditory standard stimuli elicited frontally distributed mismatch negativities (MMNs). The MMN persisted during visual attention and was unaffected by visual processing load. However, the MMN to deviants but not DEVIANTS was enhanced in amplitude with auditory attention. Attention to visual stimuli resulted in positive (Pdv, latency 70-130 msec) and negative (Ndv, 170-270 msec) modulations of visual ERPs, that increased with increasing processing load. Prominent visual deviance-related negativities were observed at occipital and infero-temporal scalp sites (latencies 90-290 msec), but only to DEVIANT visual stimuli. The early MMN-like portion of the visual deviance-related negativity was independent of attention, with equal amplitudes during different auditory and visual conditions.

Attentional modulation of human auditory cortex

Attention powerfully influences auditory perception, but little is understood about the mechanisms whereby attention sharpens responses to unattended sounds. We used high-resolution surface mapping techniques (using functional magnetic resonance imaging, fMRI) to examine activity in human auditory cortex during an intermodal selective attention task. Stimulus-dependent activations (SDAs), evoked by unattended sounds during demanding visual tasks, were maximal over mesial auditory cortex. They were tuned to sound frequency and location, and showed rapid adaptation to repeated sounds. Attention-related modulations (ARMs) were isolated as response enhancements that occurred when subjects performed pitch-discrimination tasks. In contrast to SDAs, ARMs were localized to lateral auditory cortex, showed broad frequency and location tuning, and increased in amplitude with sound repetition. The results suggest a functional dichotomy of auditory cortical fields: stimulus-determined mesial fields that faithfully transmit acoustic information, and attentionally labile lateral fields that analyze acoustic features of behaviorally relevant sounds.

A distributed cortical network for auditory sensory memory in humans

Auditory sensory memory is a critical first stage in auditory perception that permits listeners to integrate incoming acoustic information with stored representations of preceding auditory events. Here, we investigated the neural circuits of sensory memory using behavioral and electrophysiological measures of auditory processing in patients with unilateral brain damage to dorsolateral prefrontal cortex, posterior association cortex, or the hippocampus. We used a neurophysiological marker of an automatic component of sensory memory, the mismatch negativity (MMN), which can be recorded without overt attention. In comparison with control subjects, temporal-parietal patients had impaired auditory discrimination and reduced MMN amplitudes with both effects evident only following stimuli presented in the ear contralateral to the lesioned hemisphere. This suggests that auditory sensory memories are predominantly stored in auditory cortex contralateral to the ear of presentation. Dorsolateral prefrontal damage impaired performance and reduced MMNs elicited by deviant stimuli presented in either ear, implying that dorsolateral prefrontal cortices have a bilateral facilitatory effect on sensory memory storage. Hippocampal lesions did not affect either performance or electrophysiological measures. The results provide evidence of a temporal-prefrontal neocortical network critical for the transient storage of auditory stimuli.

Lesions of frontal cortex diminish the auditory mismatch negativity

Event-related brain potentials to non-attended auditory stimuli were recorded from patients with dorsolateral prefrontal cortex (DPFCx) lesions and from age-matched control subjects as they performed a visual reaction time task. Auditory stimuli consisted of monaural sequences of repetitive standard tones (1000 Hz) and occasional deviant tones of a higher frequency (1300 Hz). In comparison with control subjects, DPFCx patients showed enhanced P1 amplitudes (mean peak latency 50 msec), consistent with reduced frontally mediated gating of sensory input to the auditory cortex. The mismatch negativity (MMN) elicited by deviant tones was reduced in DPFCx patients over a broad latency range (130-210 msec), especially over the lesioned hemisphere and for tones delivered to the ear ipsilateral to the lesion. The results suggest that DPFCx and DPFCx-temporal projections play a critical role in involuntary orienting to physical changes in sequences of non-attended auditory stimuli.

The effects of frontal cortex lesions on event-related potentials during auditory selective attention

We compared electrophysiological indices of auditory selective attention in control subjects and in patients with unilateral lesions of the dorsolateral frontal lobes. In control subjects, ERPs following attended tones showed an enhanced negativity from 80 to 500 msec post-stimulus which had a different topographic distribution than the N120. Lesions of the frontal lobes reduced the attention-related negativity and impaired behavioral performance. The ERP reductions were equivalent in recordings obtained from electrodes placed over the damaged and intact cortex. A difference was noted between left and right frontal groups as a function of ear of delivery of the stimuli. Patients with left frontal lesions showed reduced attention effects following tones presented to either ear. Patients with right frontal lesions showed intact attention effects to right ear tones, but no attention-related negativity to left ear tones. When the left and right frontal groups were considered together, tones in ignored channels produced larger responses when presented to the ear contralateral to damaged cortex. These results underline the important role of the frontal lobes in processes of selective attention. Although the endogenous negativity produced in selective attention tasks does not appear to originate in dorsolateral frontal cortex, the frontal lobes exhibit a modulating influence upon it. In addition, the endogenous attention related negativity and exogenous N120 components apparently arise from different neural generators.

Generators of middle- and long-latency auditory evoked potentials: implications from studies of patients with bitemporal lesions☆

We recorded middle- and long-latency auditory evoked potentials (AEPs) in 5 patients (ages 39-72 years) with bilateral lesions of the superior temporal plane. Reconstructions of CT sections revealed that primary auditory cortex had been damaged bilaterally in four of the patients, while in the fifth an extensive left hemisphere lesion included primary auditory cortex while a right hemisphere lesion had damaged anterior auditory association areas but spared primary auditory cortex. Normal middle-latency AEPs (MAEPs) were recorded at the vertex electrode in all of the patients. In 3 of the 5 patients, MAEPs also showed normal coronal scalp distributions and were comparable in amplitude following stimulation of either ear. Two patients showed abnormalities. In one case, Na (latency 17 msec)-Pa (latency 30 msec) amplitudes were reduced over both hemispheres following stimulation of the ear contralateral to the more extensive lesion. In another, with both subcortical and cortical involvement, the Pa was abolished over the hemisphere with the more extensive lesion. Long-latency AEPs were normal in 2 patients whose lesions were largely confined to the superior temporal plane. In 2 patients with lesions extending into the inferior parietal lobe, N1s were abolished bilaterally. In the fifth patient, the N1 showed a slight reduction over the hemisphere with the more extensive lesion. Middle- and long-latency AEPs were differentially affected by some lesions. For example, patients with absent N1s could produce normal Pas. A review of these results and those of previous studies of bitemporal patients suggests that abnormalities in middle- and long-latency AEPs do not necessarily reflect damage to primary auditory cortex per se, but rather the degree of damage to adjacent areas. Abnormalities in MAEPs are associated with subcortical lesions, or cortical lesions extensive enough to denervate thalamic projection nuclei. Abnormalities in the long-latency N1 reflect lesion extension into the multi-modal areas of the inferior parietal lobule. This area appears to exert a critical modulatory influence over N1 generators outside of the superior temporal plane.

Prefrontal cortex gating of auditory transmission in humans.

Middle-latency auditory evoked potentials (MAEPs) were recorded in controls and patients with focal lesions in dorsolateral prefrontal cortex. Unilateral prefrontal lesions increased the amplitude of the Pa component of the MAEP beginning at 25-35 ms poststimulus. The data suggest that prefrontal cortex exerts early inhibitory modulation of input to primary auditory cortex in humans.

Human auditory sustained potentials. II. Stimulus relationships

The auditory sustained potential recorded from the human scalp increases in amplitude with increasing stimulus intensity. At rapid rates of stimulus presentation its amplitude decreases but proportionately less so than the amplitude of the transient onset auditory evoked potential. The frequency specificity of this rate effect is complex, suggesting that there may be two underlying components of the scalp-recorded auditory sustained potential. The amplitude of the auditory sustained potential is smaller when the tonal frequency of the stimulus is higher. With prolonged stimulus durations there is some adaptation of the amplitude of the auditory sustained potential. This potential is larger in amplitude when sounds are presented binaurally than monaurally, and has a symmetrical coronal scalp distribution that is unaffected by the ear of stimulation.