Smooth pursuit eye movement is a visual tracking reflex evoked by a smoothly moving target, usually elicited by stimuli presented on a computer monitor. The aim of the smooth pursuit reflex is to maintain the image of the moving target on the fovea, the region of the retina with the highest density of photoreceptors. The neural pathways involved in generating smooth pursuit are a complex network from the cortical visual pathways through to the brainstem ocular motor nuclei (III, IV, and VI), and consequently, an alteration in smooth pursuit performance may not in itself shed light on the actual nature of the dysfunction.

Components of smooth pursuit which are quantified include gain in the open and closed loops, as well as rates and amplitudes for both intrusive and anticipatory saccades (fast eye movements). Closed loop gain is an index of temporal synchrony of the eye and the target during pursuit, and is estimated as the ratio of the respective velocities. Open loop gain is the average acceleration during the initiation of pursuit, in the first 100ms. During this period there is no visual feedback and so the movement is solely a result of visual motion signal input. Spontaneous saccades can occur during smooth pursuit: these can either be anticipatory saccades that facilitate movement towards the target, such as reflexive visually guided saccades; or intrusive saccades, which interrupt the smooth tracking of the target, such as catch-up saccades, back-up saccades, and memory-guided saccades.

What is the evidence for eye movement dysfunction in people with bipolar disorder?

Moderate to low quality evidence suggests reaction time and error rates in antisaccade tasks are increased, and accuracy is decreased in people with bipolar disorder compared to controls. People with major depression also show increased reaction time and error rates, with no differences in accuracy. On predictive tasks, people with bipolar disorder perform similarly to controls, while people with major depression show reduced accuracy and increased correction rates. On smooth pursuit tasks, both people with bipolar disorder and depression show less pursuit gain and more initial eye accelerations and catch-up saccades. On fixation tasks, people with bipolar disorder showed more inhibition error than controls.

December 2021

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Mismatch negativity (MMN) is an auditory event-related potential that is generated when a stimulus feature deviates from the regularity of previous auditory stimuli. This deviance can be a simple physical characteristic, such as tone duration, intensity, frequency or location; or more abstract presentation characteristics, such as a lower tone in a series of ascending tones. In this way, MMN generation relies on the creation of an auditory (echoic) memory trace for the preceding tones, in order to identify the subsequent deviance. MMN is thought to be an automatic, pre-attentional process and functions as an index of auditory discrimination and echoic memory integrity. MMN is observed as the difference in ERP wave response to the standard stimuli and the deviant stimulus. Larger differences between standard and deviant stimuli and lower probability of deviant occurrence are both associated with larger MMN amplitude.

What is the evidence for mismatch negativity in people with bipolar disorder?

High quality evidence shows a medium-sized effect of reduced MMN amplitude in frontal regions of people with bipolar disorder compared to controls without any psychiatric disorder.

December 2021

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The N170 wave is an event-related brain potential measured using electroencephalography. The N170 is a negative waveform that peaks at approximately 170 msec after stimulus presentation. It is observed at occipitotemporal sites and with greater amplitude over the right hemisphere. The N170 ERP is observed in response to a variety of facial stimuli, and so it may reflect a neural mechanism for detection of human faces.

What is the evidence for N170 anomalies in people with bipolar disorder?

Low quality evidence in unclear of N170 amplitude and latency changes in people with bipolar disorder compared to controls without any psychiatric disorder.

December 2021

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The P300 wave is an event-related potential, measured using EEG. It is a spike in brain activity approximately 300ms after presentation of a target stimulus, which is usually auditory but can also be visual. A typical auditory paradigm uses a series of tones where 20% of them are infrequent targets called “oddballs”. Research subjects are asked to respond to these oddballs, and the related spike in brain activity is usually apparent in frontal regions thought to be related to contextual updating and memory storage. This is called a P3b response, while a P3a response reflects response to an infrequent non-target stimulus usually apparent in parietal regions and thought to be related to automatic attention processing. P300 measures also include amplitude and latency of the P300 wave. Amplitude is proportional to the amount of attentional resource devoted to the task and the degree of information processing required, while latency is considered a measure of stimulus classification speed.

What is the evidence for P300 anomalies in people with bipolar disorder?

Moderate to high quality evidence shows medium to large effects of reduced P300 P3b (target) amplitude and longer P300 P3b (target) latency in people with bipolar disorder compared to controls in both auditory and visual paradigms. There was also a small effect of reduced P300 P3a (non-target) amplitude in an auditory paradigm. These results remained in studies of people with or without a history of psychotic symptoms, in people with different bipolar types (bipolar I vs. bipolar II), and in people in different phases of the disorder (depression, euthymia, mania).

Compared to people with unipolar depression, there was a medium-sized effect of longer latency in people with bipolar depression, with no differences in amplitude. Results were similar in the analyses of remitted patients. The results were larger in studies conducted in China, in studies with >100 patients, and in unmedicated patients.

December 2021

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The P50 event-related potential is interpreted as a physiological substrate for an inability to “gate” or inhibit irrelevant sensory information. In this paradigm, paired auditory clicks are presented, separated by a 500ms interval. The first click initiates or conditions the inhibition, while the second (test) click indexes the strength of the inhibition. P50 ratio is quantified as the amplitude of the response to the second click divided by the first. An absence of a reduced response to the second stimulus is interpreted as a failure of inhibitory mechanisms, postulated to represent a defect in sensory gating, and a larger ratio is indicative of reduced cortical inhibition.

What is the evidence for P50 anomalies in people with bipolar disorder?

Moderate quality evidence shows large effects of increased P50 S2/S1 ratio and decreased P50 S1-S2 difference in people with bipolar disorder compared to controls. Medication may improve the S2/S1 ratio. There were no moderating effects of having or not having psychotic symptoms.

December 2021

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The startle response involves reflex behaviour intended to prepare an organism to attend to potentially relevant stimuli. The startle response can be reduced by previous presentation of a weak stimulus, and this is known as prepulse inhibition. When prepulse inhibition is high, the corresponding one-time startle response is reduced. When prepulse inhibition is low, an inability to filter out unnecessary information may occur.

Prepulse inhibition is regulated by the limbic-cortical-striatal-pallidal-thalamic neural circuit, the dopaminergic system, and by cognitive processes such as attention and emotion. Some of these processes may be dysfunctional in people with bipolar disorder.

What is the evidence for prepulse inhibition in people with bipolar disorder?

Moderate to high quality evidence suggests a small effect of reduced prepulse inhibition in people with bipolar disorder during euthymia compared to controls.

December 2021

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