Christopher D. Cowper-Smith

Neural Coding of Movement Direction in the Healthy Human Brain

Neurophysiological studies in monkeys show that activity of neurons in primary cortex (M1), pre-motor cortex (PMC), and cerebellum varies systematically with the direction of reaching movements. These neurons exhibit preferred direction tuning, where the level of neural activity is highest when movements are made in the preferred direction (PD), and gets progressively lower as movements are made at increasing degrees of offset from the PD. Using a functional magnetic resonance imaging adaptation (fMRI-A) paradigm, we show that PD coding does exist in regions of the human motor system that are homologous to those observed in non-human primates. Consistent with predictions of the PD model, we show adaptation (i.e., a lower level) of the blood oxygen level dependent (BOLD) time-course signal in M1, PMC, SMA, and cerebellum when consecutive wrist movements were made in the same direction (0° offset) relative to movements offset by 90° or 180°. The BOLD signal in dorsolateral prefrontal cortex adapted equally in all movement offset conditions, mitigating against the possibility that the present results are the consequence of differential task complexity or attention to action in each movement offset condition.

Motor IOR revealed for reaching

Inhibition of return (IOR) is a spatial phenomenon that is thought to promote visual search functions by biasing attention and eye movements toward novel locations. Considerable research suggests distinct sensory and motor flavors of IOR, but it is not clear whether the motor type can affect responses other than eye movements. Most studies claiming to reveal motor IOR in the reaching control system have been confounded by their use of peripheral signals, which can invoke sensory rather than motor-based inhibitory effects. Other studies have used central signals to focus on motor, rather than sensory, effects in arm movements but have failed to observe IOR and have concluded that the motor form of IOR is restricted to the oculomotor system. Here, we show the first clear evidence that motor IOR can be observed for reaching movements when participants respond to consecutive central stimuli. This observation suggests that motor IOR serves a more general function than the facilitation of visual search, perhaps reducing the likelihood of engaging in repetitive behavior.

Motor inhibition of return can affect prepared reaching movements.

Inhibition of return (IOR) is a widely studied phenomenon that is thought to affect attention, eye movements, or reaching movements, in order to promote orienting responses toward novel stimuli. Previous research in our laboratory demonstrated that the motor form of saccadic IOR can arise from late-stage response execution processes. In the present study, we were interested in whether the same is true of reaching responses. If IOR can emerge from processes operating at or around the time of response execution, then IOR should be observed even when participants have fully prepared their responses in advance of the movement initiation signal. Similar to the saccadic system, our results reveal that IOR can be implemented as a late-stage execution bias in the reaching control system.

Spatial Interactions between Successive Eye and Arm Movements: Signal Type Matters

Spatial interactions between consecutive movements are often attributed to inhibition of return (IOR), a phenomenon in which responses to previously signalled locations are slower than responses to unsignalled locations. In two experiments using peripheral target signals offset by 0°, 90°, or 180°, we show that consecutive saccadic (Experiment 1) and reaching (Experiment 3) responses exhibit a monotonic pattern of reaction times consistent with the currently established spatial distribution of IOR. In contrast, in two experiments with central target signals (i.e., arrowheads pointing at target locations), we find a non-monotonic pattern of reaction times for saccades (Experiment 2) and reaching movements (Experiment 4). The difference in the patterns of results observed demonstrates different behavioral effects that depend on signal type. The pattern of results observed for central stimuli are consistent with a model in which neural adaptation is occurring within motor networks encoding movement direction in a distributed manner.

Refractory effects of the N1 event-related potential in experienced cochlear implant patients

Abstract Objective: To determine if cochlear implant (CI) patients exhibit a temporal processing deficit for auditory stimuli, by examining refractory effects of the N1 event-related potential (ERP) component. Design: CI patients and normally-hearing controls were tested in an auditory refractory period paradigm while ERP recordings were collected across the scalp. Participants were presented with brief 500-Hz tones that were separated by inter-stimulus intervals (ISIs) of 500, 1000, or 3000 ms. The amplitude of the N1 was examined as a function of ISI within each group. Study sample: Ten adult CI patients and 13 age-matched normally-hearing controls were tested. Patients had long-lasting severe or profound sensorineural hearing loss prior to implantation, and a minimum of two years experience with CI activation. Results: Unlike normally-hearing controls, CI users showed no refractory effect for tones at 500 ms ISIs compared to 1000 ms. However, similar to controls, recovery from refractoriness was observed in anterior locations at 3000 ms. Conclusion: The refractory period threshold, defined as the minimum ISI where different N1 amplitudes are elicited, is greater than 1000 ms in CI patients; at least double that of normally-hearing controls.

Changes in trunk orientation do not induce asymmetries in covert orienting.

We explored the effect of trunk orientation on responses to visual targets in five experiments, following work suggesting a disengage deficit in covert orienting related to changes in the trunk orientation of healthy participants. In two experiments, participants responded to the color of a target appearing in the left or right visual field following a peripheral visual cue that was informative about target location. In three additional experiments, participants responded to the location (left/right) of a target using a spatially compatible motor response. In none of the experiments did trunk orientation interact with spatial-cuing effects, suggesting that orienting behavior is not affected by the rotation of the body relative to the head. Theoretical implications are discussed.

Saccadic inhibition of return can arise from late-stage execution processes

Inhibition of return (IOR) is thought to improve the efficiency of visual search behaviour by biasing attention, eye movements, or both, toward novel stimuli. Previous research suggests that IOR might arise from early sensory, attentional or motor programming processes. In the present study, we were interested in determining if IOR could instead arise from processes operating at or during response execution, independent from effects on earlier processes. Participants made consecutive saccades (from a common starting location) to central arrowhead stimuli. We removed the possible contribution of early sensory/attentional and motor preparation effects in IOR by allowing participants to fully prepare their responses in advance of an execution signal. When responses were prepared in advance, we continued to observe IOR. Our data therefore provide clear evidence that saccadic IOR can result from an execution bias that might arise from inhibitory effects on motor output neurons, or alternatively from late attentional engagement processes.

Directional interactions between current and prior saccades

One way to explore how prior sensory and motor events impact eye movements is to ask someone to look to targets located about a central point, returning gaze to the central point after each eye movement. Concerned about the contribution of this return to center movement, Anderson et al. (2008) used a sequential saccade paradigm in which participants made a continuous series of saccades to peripheral targets that appeared to the left or right of the currently fixated location in a random sequence (the next eye movement began from the last target location). Examining the effects of previous saccades (n−x) on current saccade latency (n), they found that saccadic reaction times (RT) were reduced when the direction of the current saccade matched that of a preceding saccade (e.g., two left saccades), even when the two saccades in question were separated by multiple saccades in any direction. We examined if this pattern extends to conditions in which targets appear inside continuously marked locations that provide stable visual features (i.e., target “placeholders”) and when saccades are prompted by central arrows. Participants completed 3 conditions: peripheral targets (PT; continuous, sequential saccades to peripherally presented targets) without placeholders; PT with placeholders; and centrally presented arrows (CA; left or right pointing arrows at the currently fixated location instructing participants to saccade to the left or right). We found reduced saccadic RT when the immediately preceding saccade (n−1) was in the same (vs. opposite) direction in the PT without placeholders and CA conditions. This effect varied when considering the effect of the previous 2–5 (n−x) saccades on current saccade latency (n). The effects of previous eye movements on current saccade latency may be determined by multiple, time-varying mechanisms related to sensory (i.e., retinotopic location), motor (i.e., saccade direction), and environmental (i.e., persistent visual objects) factors.