Case studies in neuroscience: Cortical contributions to the frequency-following response depend on subcortical synchrony

Frequency-following responses to musical notes spanning the octave 65-130 Hz were elicited in a person with auditory neuropathy, a disorder of subcortical neural synchrony, and a control subject. No phaselocked responses were observed in the person with auditory neuropathy. The control subject had robust responses synchronized to the fundamental frequency and its harmonics. Cortical onset responses to each note in the series were present in both subjects. These results support the hypothesis that subcortical neural synchrony is necessary to generate the frequency-following response-including for stimulus frequencies at which a cortical contribution has been noted. Although auditory cortex ensembles may synchronize to fundamental frequency cues in speech and music, subcortical neural synchrony appears to be a necessary antecedent.

Oculomotor Target Selection is Cortically Mediated by Complex Objects

Successful oculomotor target selection often requires discriminating visual features but it remains contentious whether oculomotor substrates encoding saccade vectors functionally contribute to this process. One possibility is that visual features are discriminated cortically and oculomotor modules select the object with the highest activation in the set of all preprocessed cortical object representations, while an alternative possibility is that oculomotor modules actively discriminate potential targets based on visual features. If the latter view is correct, these modules should not require input from specialized visual cortices encoding the task relevant features. We therefore examined whether the latency of visual onset responses elicited by abrupt distractor onsets is consistent with input from specialized visual cortices by non-invasively measuring human saccade metrics (saccade curvature, endpoint deviations, saccade frequency, error proportion) as a function of distractor processing time for novel, visually complex distractors that had to be discriminated from a target to guide saccades. Visual onset response latencies were ~110 ms, consistent with projections from anterior cortical sites specialized for object processing. Surprisingly, oculomotor visual onset responses encoded features, as we manipulated the visual similarity between targets and distractors and observed an increased visual onset response magnitude and duration when the distractor was highly similar to the target, which was not attributable to an inhibitory processing delay. Visual onset responses were dynamically modulated by executive function, as these responses were anticipatorily extinguished over the time course of the experiment. As expected, the latency of distractor-related inhibition was modulated by the behavioral relevance of the distractor.Significance StatementWe provide novel insights into the role of the oculomotor system in saccadic target selection by challenging the convention that neural substrates that encode oculomotor vectors functionally contribute to target discrimination. Our data show that the oculomotor system selects a winner from amongst the preprocessed object representations output from specialized visual cortices as supposed to discriminating visual features locally. We also challenge the convention that oculomotor visual onset responses are feature-invariant, as they encoded task-relevance.

Happy you, happy me: expressive changes on a stranger’s voice recruit faster implicit processes than self-produced expressions

In social interactions, people have to pay attention both to the what and who. In particular, expressive changes heard on speech signals have to be integrated with speaker identity, differentiating e.g. self- and other-produced signals. While previous research has shown that self-related visual information processing is facilitated compared to non-self stimuli, evidence in the auditory modality remains mixed. Here, we compared electroencephalography (EEG) responses to expressive changes in sequence of self- or other-produced speech sounds, using a mismatch negativity (MMN) passive oddball paradigm. Critically, to control for speaker differences, we used programmable acoustic transformations to create voice deviants which differed from standards in exactly the same manner, making EEG responses to such deviations comparable between sequences. Our results indicate that expressive changes on a stranger’s voice are highly prioritized in auditory processing compared to identical changes on the self-voice. Other-voice deviants generate earlier MMN onset responses and involve stronger cortical activations in a left motor and somatosensory network suggestive of an increased recruitment of resources for less internally predictable, and therefore perhaps more socially relevant, signals.

Onset responses of ventilation and cerebral blood flow to hypercapnia in humans: rest and exercise

The respiratory and cerebrovascular reactivity to changes in arterial Pco2 ([Formula: see text]) is an important mechanism that maintains CO2 or pH homeostasis in the brain. It remains unclear, however, how cerebrovascular CO2 reactivity might influence the respiratory chemoreflex. The purpose of the present study was therefore to examine the interaction between onset responses of the respiratory chemoreflex and middle cerebral artery (MCA) mean blood velocity ( Vmean) to hypercapnia (5.0% CO2-40% O2-balance N2) at rest and during dynamic exercise (∼1.0 l/min O2 consumption). Each onset response was evaluated using a single-exponential regression model consisting of the response time latency [CO2-response delay ( t0)] and time constant (τ). At rest, t0 and τ data indicated that the MCA Vmean onset response was faster than the ventilatory (V̇e) response ( P < 0.001). In contrast, during exercise, t0 of V̇e and MCA Vmean onset responses were decreased. In addition, despite the enhanced [Formula: see text] response to CO2 administration ( P = 0.014), τ of MCA Vmean tended to increase during exercise ( P = 0.054), whereas τ of V̇e decreased ( P = 0.015). These findings indicate that 1) at rest, faster washout of CO2 via cerebral vasodilation results in a reduced activation of the central chemoreflex and subsequent reduced V̇e onset response, and 2) during exercise, despite higher rates of increasing [Formula: see text], the lack of change in the onset response of cerebral blood flow and reduced washout of CO2 may act to augment the V̇e onset response.