Adaptation of high-gamma responses in human auditory association cortex

This study investigates adaptation of high-frequency cortical responses [>60 Hz; high-gamma (HG)] to simple and complex sounds in human nonprimary auditory cortex. We used intracranial electrocorticographic recordings to measure event-related changes in HG power as a function of stimulus probability. Tone and speech stimuli were presented in a series of traditional oddball and control paradigms. We hypothesized that HG power attenuates with stimulus repetition over multiple concurrent time scales in auditory association cortex. Time-frequency analyses were performed to identify auditory-responsive sites. Single-trial analyses and quantitative modeling were then used to measure trial-to-trial changes in HG power for high (frequent), low (infrequent), and equal (control) stimulus probabilities. Results show strong reduction of HG responses to frequently repeated tones and speech, with no differences in responses to infrequent and equal-probability stimuli. Adaptation of the HG frequent response, and not stimulus-acoustic differences or deviance-detection enhancement effects, accounted for the differential responses observed for frequent and infrequent sounds. Adaptation of HG responses showed a rapid onset (less than two trials) with slower adaptation between consecutive, repeated trials (2–10 s) and across trials in a stimulus block (∼7 min). The auditory-evoked N100 response also showed repetition-related adaptation, consistent with previous human scalp and animal single-unit recordings. These findings indicate that HG responses are highly sensitive to the regularities of simple and complex auditory events and show adaptation on multiple concurrent time scales in human auditory association cortex.

The Speed of Visual Search in the Absence of Sensory Effects

The purpose of this study was to investigate the efficiency or speed of the frequently used L versus T visual search when sensory effects were controlled, ie ‘set size’ was not defined as the number of distractor patterns, but the number of distractors in the display was kept constant and the number of possible target positions varied. A search display consisted of an L-target among T-distractors, and the observer's task was to report the presence or absence of the target (experiment 1) or to identify it (whether the L-target was left-facing or right-facing; experiment 2). The observer was instructed prior to each stimulus block, about the display locations in which the target could appear. In both experiments, search time increased significantly with an increasing number of possible target locations, thus indicating that L versus T search is not ‘serial’ owing to sensory effects. Because, in the first two experiments, a search display was visible until the observer gave a response, ‘serial’ search might have resulted just from eye movements. Therefore, a control experiment was run in which display duration was limited to 150 ms. The results of this experiment showed that, even when eye movements were prevented, the search still occurred ‘serially’, ie response time increased as a function of the number of possible target positions.

Cardiac Response to Internally and Externally Elicited Stressful Imagery as a Function of Field Dependence

The present study investigated heart-rate response to internally and externally elicited stressful imagery as a function of field dependence. 40 female undergraduates were divided into five field-dependent groups. Both the internal and the external modes of presentation included a stimulus block followed by an image block. The internal-stimulus block was an internally elicited thought, while the external-stimulus block was a visually presented typewritten statement. Half of the Ss within each field-dependent group experienced the external mode and half experienced the internal mode. A tone served as the time-locked cue to signal the thoughts and the images. The results indicated that the internal mode produced significantly greater heart-rate response than the external mode, and there was a trend toward field-independent Ss reacting with higher heart rate to both modes. These findings and trends are discussed in terms of the importance of internally elicited cognitive activity in the experience of fear, the Lacey hypothesis of directional fractionation, and the relationship between field dependence and various autonomic and personality characteristics.