Physiology of Active Vision

Visual Perception and Motor Control

Description of visual perception outside a car windshield — Eye movements are used to scan both far and near objects in our environment

Research in the Hafed lab aims to investigate the neural mechanisms through which visual perception interacts with motor control. We employ techniques for monitoring and focally perturbing neural activity, coupled with computational neuroscience and careful behavioral control, to understand the functional contribution of individual brain circuits in coordinating perception and action. Besides clarifying our understanding of the sense of vision, our research also sheds light on how neural activity that is distributed across multiple brain areas is organized to support behavior.

Research Goals

Humans are tremendously reliant on vision to interact with their environment. Such interaction often involves the generation of motor outputs in response to visual stimuli, but these outputs themselves can alter the sensory inputs used by the visual system. This is particularly true of eye movements, which are necessary for high-acuity vision but at the same time cause the largest and most frequent disruptions to the retinal images projected from the eyes to the brain. Our primary focus is on understanding how eye movements may serve and influence visual perception.

Role and Genesis of Fixational Eye Movements

Whenever we maintain steady gaze at an object of interest, our eyes never remain still. Instead, they undergo several types of ‘fixational’ movements, including ones designated as ‘microsaccades’ and others referred to as ‘drifts’. Microsaccades are extremely small versions of the saccadic eye movements that we employ in everyday life to scan our visual environment, but they never displace retinal images of the objects we are looking at away from the fovea; drifts are slow meandering movements of the eyes between two successive saccades or microsaccades. Even though microsaccades and drifts were discovered several decades ago, the brain mechanisms for generating and controlling them remain largely unknown. Through the use of neuronal recording, reversible activation or inactivation of subsets of neuronal populations, and computer simulations, we are investigating the mechanisms for generating these eye movements. One of our long-term goals is to uncover the full extent of brain control over fixational eye movements and when/why different types of eye movements may be triggered.

Perceptual Stability in the Face of Eye Movements

Eye movements, no matter how small, displace retinal images whenever they occur. However, simple introspection reveals that we do not perceptually experience moving images whenever we move our eyes and bodies. Thus, understanding any changes in sensory processing that may arise as a result of self-motion is important for understanding how we continue to perceive a stable environment despite being active organisms. To address this question, we focus on interactions between fast or slow eye movements and neural visual sensitivity in multiple brain areas.

Eye Movements and Cognition/Perception

We design and employ behavioral tasks that use more complex visual stimuli than the minimalist spots, which are overwhelmingly common in oculomotor research. Similar to scenes in many natural human perceptual experiences, such visual stimuli require a transfer of information from the sensory domain to the motor domain and therefore allow us to ask important questions about the neural mechanisms underlying such transformations. For example, one such question relates to the function of the superior colliculus (SC) and how it supports orienting behaviors. Using a combination of experimental and modeling techniques, we are recasting traditional views of the SC to provide a new conceptual framework for understanding this structure's involvement in higher-level sensory and cognitive processes. This is serving as an important basis for understanding how eye movements (including fixational eye movements) interact with cognition.

Selected Publications

Find our full list of publications here

Willeke, K. F., Cardenas, A. R., Bellet, J., & Hafed, Z. M. (2022). Severe distortion in the representation of foveal visual image locations in short term memory. Proceedings of the National Academy of Sciences, Vol. 119, No. 24, e2121860119. doi: 10.1073/pnas.2121860119.
Chen, C.-Y., Hoffmann, K.-P., Distler, C., & Hafed, Z. M. (2019). The foveal visual representation of the primate superior colliculus. Current Biology, Vol. 29, pp. 2109-2119, https://doi.org/10.1016/j.cub.2019.05.040.
Hafed, Z. M. (2013). Alteration of visual perception prior to microsaccades. Neuron, Vol. 77, pp. 775-786.
Hafed, Z. M., Goffart, L., & Krauzlis, R. J. (2009). A neural mechanism for microsaccade generation in the primate superior colliculus. Science, Vol. 323, No. 5916, pp. 940-943.
Hafed, Z. M. & Clark, J. J. (2002). Microsaccades as an overt measure of covert attention shifts. Vision Research, Vol. 42, No. 22, pp. 2533-2545.

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