Showing papers by "Gregory McCarthy published in 1996"

Electrophysiological studies of face perception in humans

Abstract: Event-related potentials (ERPs) associated with face perception were recorded with scalp electrodes from normal volunteers. Subjects performed a visual target detection task in which they mentally counted the number of occurrences of pictorial stimuli from a designated category such as butterflies. In separate experiments, target stimuli were embedded within a series of other stimuli including unfamiliar human faces and isolated face components, inverted faces, distorted faces, animal faces, and other nonface stimuli. Human faces evoked a negative potential at 172 msec (N170), which was absent from the ERPs elicited by other animate and inanimate nonface stimuli. N170 was largest over the posterior temporal scalp and was larger over the right than the left hemisphere. N170 was delayed when faces were presented upside-down, but its amplitude did not change. When presented in isolation, eyes elicited an N170 that was significantly larger than that elicited by whole faces, while noses and lips elicited small negative ERPs about 50 msec later than N170. Distorted human faces, in which the locations of inner face components were altered, elicited an N170 similar in amplitude to that elicited by normal faces. However, faces of animals, human hands, cars, and items of furniture did not evoke N170. N170 may reflect the operation of a neural mechanism tuned to detect (as opposed to identify) human faces, similar to the “structural encoder” suggested by Bruce and Young (1986). A similar function has been proposed for the face-selective N200 ERP recorded from the middle fusiform and posterior inferior temporal gyri using subdural electrodes in humans (Allison, McCarthy, Nobre, Puce, & Belger, 1994c). However, the differential sensitivity of N170 to eyes in isolation suggests that N170 may reflect the activation of an eye-sensitive region of cortex. The voltage distribution of N170 over the scalp is consistent with a neural generator located in the occipitotemporal sulcus lateral to the fusiform/inferior temporal region that generates N200.

Differential sensitivity of human visual cortex to faces, letterstrings, and textures: a functional magnetic resonance imaging study.

Abstract: Twelve normal subjects viewed alternating sequences of unfamiliar faces, unpronounceable nonword letterstrings, and textures while echoplanar functional magnetic resonance images were acquired in seven slices extending from the posterior margin of the splenium to near the occipital pole. These stimuli were chosen to elicit initial category-specific processing in extrastriate cortex while minimizing semantic processing. Overall, faces evoked more activation than did letterstrings. Comparing hemispheres, faces evoked greater activation in the right than the left hemisphere, whereas letterstrings evoked greater activation in the left than the right hemisphere. Faces primarily activated the fusiform gyrus bilaterally, and also activated the right occipitotemporal and inferior occipital sulci and a region of lateral cortex centered in the middle temporal gyrus. Letterstrings primarily activated the left occipitotemporal and inferior occipital sulci. Textures primarily activated portions of the collateral sulcus. In the left hemisphere, 9 of the 12 subjects showed a characteristic pattern in which faces activated a discrete region of the lateral fusiform gyrus, whereas letterstrings activated a nearby region of cortex within the occipitotemporal and inferior occipital sulci. These results suggest that different regions of ventral extrastriate cortex are specialized for processing the perceptual features of faces and letterstrings, and that these regions are intermediate between earlier processing in striate and peristriate cortex, and later lexical, semantic, and associative processing in downstream cortical regions.

Activation of Human Prefrontal Cortex during Spatial and Nonspatial Working Memory Tasks Measured by Functional MRI

Abstract: Separate working memory domains for spatial location, and for objects, faces, and patterns, have been identified in the prefrontal cortex (PFC) of nonhuman primates. We have used functional magnetic resonance imaging to examine whether spatial and nonspatial visual working memory processes are similarly dissociable in human PFC. Subjects performed tasks which required them to remember either the location or shape of successive visual stimuli. We found that the mnemonic component of the working memory tasks affected the hemispheric pattern of PFC activation. The spatial (LOCATION) working memory task preferentially activated the middle frontal gyrus (MFG) in the right hemisphere, while the nonspatial (SHAPE) working memory task activated the MFG in both hemispheres. Furthermore, the area of activation in the left hemisphere extended into the inferior frontal gyrus for nonspatial SHAPE task. A perceptual target (DOT) detection task also activated the MFG bilaterally, but at a level approximately half that of the working memory tasks. The activation in the MFG occurred within 3-6 s of task onset and declined following task offset. Time-course analysis revealed a different pattern for cingulate gyrus, in which activation occurred upon task completion. Cingulate gyrus activation was greatest following the SHAPE task and was greater in the left hemisphere. The present results support the prominent role of the PFC and, specifically, the MFG in working memory, and indicate that the mnemonic content of the task affects the relative weighting of hemispheric activation.

Magnetic resonance imaging studies of functional brain activation: analysis and interpretation.

Abstract: We have demonstrated that a time series of echoplanar images can contain low frequency noise components which confound analysis of functional MRI data. In simulated tasks of long duration, the false positive rate from t-test analyses greatly exceeded the statistical probability level. As task durations were shortened, the false positive rate declined. We also demonstrated that voxels representing extensive regions of the brain covary significantly over time. This covariation challenges the independence assumption of t-test and other analytical procedures and likely contributes to the false positive rate. The frequency spectra of many voxels showed relatively little power at higher frequencies with the important exception of some blood vessels (Fig. 12). Experimental designs in which stimulus or task conditions were alternated at these higher frequencies (e.g. 0.083 Hz corresponding to a 6 sec task duration and a 12 sec period for a complete two task cycle) did not show an inflated false positive rate when analyzed by t-test. We used the alternating tasks design with task durations of 8.73 sec, 6.4 sec, and 6.0 sec coupled with a frequency domain analysis strategy in a series of somatosensory, motor, perceptual, and working memory experiments. This combination of design and analysis was successful in identifying reliable activations across groups of subjects with a minimum of apparently spurious activations. By introducing a 180 degrees phase shift by reversing task order, we have been able to eliminate the contribution of most high frequency noise sources (such as large blood vessels). By segregating low frequency noise from the frequency of stimulus alternation, we routinely generate stable results in the presence of low frequency noise and drift. Despite the usefulness of the rapid task alternation and frequency domain techniques demonstrated here, there are potential problems and limitations in their application: 1. The short duration of our tasks results in an approximately sinusoidal activation waveform. With longer duration tasks, the activation time course would appear more square with a more complex frequency spectrum than the single peak demonstrated above. In such circumstances we have used convolution analysis with an expected waveform (McCarthy et al. 1996), similar to the approach of Bandettini et al. (1993). 2. If the activation in one task condition is significantly delayed and extends well into the period of the second task, it will be difficult to determine which task produced the activation. This problem is not specific to frequency analysis, and would occur as well for t-tests. One solution we have used is running a single active task against a relatively neutral control such as fixation to determine the usual activation dynamics of the active task. 3. Common activations by two alternating tasks are de-emphasized. This problem is also not specific to frequency analysis, and in most circumstances is an advantage rather than a disadvantage. However, if uncertain as to whether a task is capable of producing any activation, we have again used the strategy of running the task against a relatively neutral control. 4. Some tasks do not lend themselves to the short durations used here. 5. The frequency domain procedures used are conservative and may underestimate the true anatomical extent of the activation. In practice we compute t-tests in addition to the frequency domain techniques to guard against this possibility. Many of the advantages of the procedures described here are due to the alternation of short duration tasks rather than the application of frequency domain techniques per se. However, the success of these techniques in isolating periodic task-related signal changes suggest that a more complex design with concurrent stimulation presented at different frequencies might be feasible. Such designs may have advantages in that categories of stimuli would not be presented in isolation but against a changing ba