Biomedical subjects
H C Hughes
Publications and source records attributed to H C Hughes.
Effect of uncertainty on the time course for selection of verbal name codes.
Response selection takes time. Hick's law (Hick, 1952) predicts that the time course of response selection is a logarithmic function of the number of equally likely response alternatives. However, recent work has shown that oculomotor responses constitute noteworthy exceptions in that the latencies of saccades (Kveraga, Boucher, & Hughes, 2002) and smooth pursuit movements (Berryhill, Kveraga, Boucher, & Hughes, 2004) are completely independent of response uncertainty. This finding extends to the case in which the required response was known in advance (i.e., simple reaction times [RTs] were equivalent to choice RTs). In view of these results, we reevaluated reports that latencies to name visually presented digits (Experiment 1) and/or repeat aurally presented digits (Experiment 2) are similarly independent of the size of the response set. We found that naming latencies were equivalent for response set sizes from one to eight, but simple RTs (response set of one) were faster. Thus, the overlearned task of digit naming is indeed highly automatic but has not reached the level of automaticity characteristic of the oculomotor system.
Responses of human auditory association cortex to the omission of an expected acoustic event.
Unexpected auditory events initiate a complex set of event-related potentials (ERPs) that vary in their latency and anatomical localization. Such "mismatch" responses include active responses to the omission of an expected event or the omission of elements in expected stimulus composites. Here we describe intracranial recordings of middle-latency ERPs elicited by the omission of an auditory event. We first presented a sequence of tones at regular temporal intervals and the tone was omitted 20% of the time. In a second condition, we presented a sequence of tone pairs and the second tone of the pair was omitted 20% of the time. These two conditions are complementary in that the single tone conformed to the expectancy in one condition, but violated the expectancy in the other. All patients demonstrated localized cortical responses to missing tones that were topographically similar to the responses evoked by actual tones. Responses to both actual and omitted tones were observed bilaterally in the vicinity of the temporal--parietal junction, where we also obtained midlatency ERPs to a variety of other auditory stimuli. Responses that appeared to be selective for the nonoccurrence of expected tones were also observed in a number of subjects. We interpret these effects in terms of processes associated with the comparison of sensory inputs to the contents of a short-term auditory memory. Such a system could automatically detect deviant auditory events, and provide input to higher-level, task-dependent cognitive processes.
Ocular pursuit of predicted motion trajectories.
The initiation and maintenance of slow eye movements (SEMs) usually depend on the perception of a moving stimulus. However, the endogenous representation of predictable target motion can be sufficient to initiate and maintain brief episodes of SEM even when the stimulus is not present. In this note, we show that expectancies generated by predictable stimulus motion trajectories can also produce smooth deceleration, reversal of direction, and subsequent acceleration in these movements, and explore the limits of the predictive component of the SEM control system quantitatively.
Afferent delays and the mislocalization of perisaccadic stimuli.
Determining the precise moment a visual stimulus appears is difficult because visual response latencies vary. This temporal uncertainty could cause localization errors to brief visual targets presented before and during eye movements if the oculomotor system cannot determine the position of the eye at the time the stimulus appeared. We investigated the effect of varying neural processing time on localization accuracy for perisaccadic visual targets that differed in luminance. Although systematic errors in localization were observed, the effect of luminance was surprisingly small. We explore several hypotheses that may explain why processing delays are not more disruptive to localization performance.
Preoperative functional mapping using intracranial EEG activation methods.
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A quantifiable model of axonal regeneration in the demyelinated adult rat spinal cord.
Strategies to increase the extent of axonal regeneration in the adult CNS must address an array of intrinsic and environmental factors which influence neuritic outgrowth. In order to develop an in vivo model of axonal regeneration in which potential therapies may be assessed, we have quantified growth cones within demyelinated regions in the dorsal funiculus of the spinal cord, following a discrete axotomy. Demyelinated lesions were produced by the intraspinal injection of galactocerebroside antibodies plus serum complement proteins. Axonal integrity was not compromised by the demyelination protocol. Axonal injury was induced at the caudal extent of the demyelinated region using a micromanipulator-controlled Scouten knife. The severity of axonal injury was varied in different animals at the time of surgery and was quantified 8 days later by counting degenerate axons in transverse 1-microm resin sections. Evidence of axonal regeneration within these animals was assessed by an electron microscopic analysis of growth cone frequency and position relative to the site of axotomy. Growth cones were identified within the region of demyelination only; no growth cones were identified within the dorsal column white matter adjacent to the demyelinated region, or rostral or caudal to the region of demyelination, or in animals with an injury but no demyelination. Quantification of growth cones within regions of demyelination indicated a strong linear relationship (P < 0.001) between the number of growth cones and the number of axons severed. These findings indicate that demyelination facilitates axonal regeneration in the adult rat CNS and illustrate a quantifiable method of assessing axonal regeneration.
Spatial characteristics of visual-auditory summation in human saccades.
Bimodal (auditory + visual) stimuli reduce saccade latencies in human observers to a degree that exceeds levels predictable by probabilistic summation between parallel, independent unimodal pathways. These interactions have been interpreted in terms of converging visual and auditory afferents within the oculomotor pathways, specifically within the superior colliculus (SC). The present work describes the spatial tuning of auditory-visual summation in human saccades, using diagnostics derived from stochastic models of information processing. Consistent with expectations based on the electrophysiology of the SC, the magnitude of facilitation varied with the degree of spatial correspondence, and the spatial tuning was quite coarse.
Advances in cubicle design using computational fluid dynamics as a design tool.
As part of a recent animal facility refurbishment, a cubicle containment system was designed to increase the amount of experimental space and also provide containment facilities to support the holding and use of specialized animal models. In order to achieve this, a series of computational fluid dynamic (CFD) studies was undertaken to evaluate the effects of different airflows and in order to optimize ventilation, a variety of exhaust/supply arrangements and animal loads was employed. These studies showed that air delivered via two, opposed, low level ducts, at a rate of 20 air changes per hour and exhausted high in the cubicle above the rack, was the optimal configuration resulting in minimal turbulence, stagnation and entrainment.
Spatial scale interactions and visual-tracking performance.
It has previously been observed that low spatial frequencies (< or = 1.0 cycles deg-1) tend to dominate high spatial frequencies (> or = 5.0 cycles deg-1) in several types of visual-information-processing tasks. This earlier work employed reaction times as the primary performance measure and the present experiments address the possibility of low-frequency dominance by evaluating visually guided performance of a completely different response system: the control of slow-pursuit eye movements. Slow-pursuit gains (eye velocity/stimulus velocity) were obtained while observers attempted to track the motion of a sine-wave grating. The drifting gratings were presented on three types of background: a uniform background, a background consisting of a stationary grating, or a flickering background. Low-frequency dominance was evident over a wide range of velocities, in that a stationary high-frequency component produced little disruption in the pursuit of a drifting low spatial frequency, but a stationary low frequency interfered substantially with the tracking of a moving high spatial frequency. Pursuit was unaffected by temporal modulation of the background, suggesting that these effects are due to the spatial characteristics of the stationary grating. Similar asymmetries were observed with respect to the stability of fixation: active fixation was less stable in the presence of a drifting low frequency than in the presence of a drifting high frequency.
Effects of caging type and group size on selected physiologic variables in rats.
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The influence of position and orientation of racks on airflow dynamics in a small animal room.
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Express saccades in cat: effects of task and target modality.
Saccadic eye movements to visual, auditory, and bimodal targets were measured in four adult cats. Bimodal targets were visual and auditory stimuli presented simultaneously at the same location. Three behavioral tasks were used: a fixation task and two saccadic tracking tasks (gap and overlap task). In the fixation task, a sensory stimulus was presented at a randomly selected location, and the saccade to fixate that stimulus was measured. In the gap and overlap tasks, a second target (hereafter called the saccade target) was presented after the cat had fixated the first target. In the gap task, the fixation target was switched off before the saccade target was turned on; in the overlap task, the saccade target was presented before the fixation target was switched off. All tasks required the cats to redirect their gaze toward the target (within a specified degree of accuracy) within 500 ms of target onset, and in all tasks target positions were varied randomly over five possible locations along the horizontal meridian within the cat's oculomotor range. In the gap task, a significantly greater proportion of saccadic reaction times (SRTs) were less than 125 ms, and mean SRTs were significantly shorter than in the fixation task. With visual targets, saccade latencies were significantly shorter in the gap task than in the overlap task, while, with bimodal targets, saccade latencies were similar in the gap and overlap tasks. On the fixation task, SRTs to auditory targets were longer than those to either visual or bimodal targets, but on the gap task, SRTs to auditory targets were shorter than those to visual or bimodal targets. Thus, SRTs reflected an interaction between target modality and task. Because target locations were unpredictable, these results demonstrate that cats, as well as primates, can produce very short latency goal-directed saccades.
Effects of warning signals and fixation point offsets on the latencies of pro- versus antisaccades: implications for an interpretation of the gap effect.
The present study was designed to evaluate whether fixation point offsets have the same effects on the average latencies of prosaccades (responses towards target) and antisaccades (responses away from target). Gap and overlap conditions were run with and without an acoustic warning signal. The 'gap effect' was taken to be the difference in mean reaction time between gap and overlap trials. This effect was dramatically reduced by the presentation of the warning signal. Without this signal, fixation offsets can serve as warning signals themselves, which artifactually inflates the magnitude of the gap effect. The warning effect of fixation offsets was equivalent for pro and antisaccades. A significant gap effect is still evident with the acoustic warning signal; however, in this case it is associated primarily with prosaccades. These results replicate and extend our previous work demonstrating that, if their warning effects are controlled, the facilitatory effects of fixation point offsets are response dependent, and suggesting the existence of a component process (fixation release) which is closely linked with the processing architecture underlying target-directed saccades.
Fate of neglected targets: a chronometric analysis of redundant target effects in the bisected brain.
The authors examined some of the sensorimotor effects of the split-brain operation to understand how a "dual mind" can produce unified behavior. They report psychophysical evidence of extinction to bilateral simultaneous stimulation in callosotomy patient J.W.: Although he could verbally report the occurrence of a unilateral left or right visual field target, left field report accuracy dropped by 34% when targets occurred bilaterally. Paradoxically, the same stimulus conditions produced abnormally robust redundant signal effects on simple manual and vocal reaction times, which exceeded predictions that were based on probability summation. Neural summation is often inferred from redundancy gain of this magnitude. Because this seems less likely after callosotomy, the authors suggest a model that is based on response competition between the disconnected hemispheres to account for J.W.'s redundant target effects. The dissociation between explicit report and motor performance is discussed.
The use of computational fluid dynamics for modeling airflow design in a kennel facility.
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Diagnosis of a case of Mycobacterium tuberculosis in a cynomolgus (Macaca fascicularis) monkey colony by polymerase chain reaction and enzyme-linked immunosorbent assay.
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Parallel and serial processes in the human oculomotor system: bimodal integration and express saccades.
Saccadic reaction times (SRTs) were analyzed in the context of stochastic models of information processing (e.g., Townsend and Ashby 1983) to reveal the processing architecture(s) underlying integrative interactions between visual and auditory inputs and the mechanisms of express saccades. The results support the following conclusions. Bimodal (visual+auditory) targets are processed in parallel, and facilitate SRT to an extent that exceeds levels attainable by probability summation. This strongly implies neural summation between elements responding to spatially aligned visual and auditory inputs in the human oculomotor system. Second, express saccades are produced within a separable processing stage that is organized in series with that responsible for intersensory integration. A model is developed that implements this combination of parallel and serial processing. The activity in parallel input channels is summed within a sensory stage which is organized in series with a pre-motor and motor stage. The time course of each subprocess is considered a random variable, and different experimental manipulations can selectively influence different stages. Parallels between the model and physiological data are explored.