Psych 129 - Sensory Processes
Retinal images are dynamic
Temporal response of the visual system
- Images contain motion due to:
1. movement of the eyes
2. movement of the head/body (ego-motion)
3. movement of things in the world
- Images are completely defined as luminance fluctuations as a function of space, wavelength, and time:
Computation of motion
- The Flicker fusion frequency, or critical flicker frequency (CFF), characterizes the temporal response of the visual system. This is the frequency at which a flashing dot no longer appears to flashi.e., it appears constant. For most humans this is about 60 Hz. (for honeybees it is around 300 Hz!)
- The visual system is most sensitive to flicker around 10 Hz.
- Film is 24 Hz (but really 48 since each frame is shown twice).
- If an image is tachistoscopically presented (i.e., extremely briefly), you can still recognize it because the photoreceptors and other neurons within the visual system will hold on to it for up to 100 ms. You can erase this residual image by masking it with another image that is presented immediately (i.e., within 20-40 ms) afterwards. But oftentimes, even if the subject claims they didnt see anything in the first image, they will show evidence through other tasks that they did in some sense see what was there - i.e., subliminal perception.
- The computation of motion requires establishing a correspondence between images across time (analogous to the way stereopsis relies on comparison of images across space).
- The Reichardt detector is a model for how neurons compute motion by comparing pixels across time.
- The aperture problem arises in the computation of motion because locally (for example, within the receptive field of a V1 neuron) the direction of motion is ambiguous. Establishing the true motion of an object requires pooling across many local motion mechanisms. When this pooling process and the assumptions upon which it is built go awry, you get motion illusions. The so-called Barber pole illusion is such an example.
- Another type of motion illusion occurs due to adaptation. The so-called waterfall illusion occurs when you stare at something moving in the same direction for awhile (i.e., about a minute) and then look at something stationary immediately afterwards. The stationary object will appear to move in the opposite direction of the adapting stimulus. You can explain such motion after-effects in terms of distributed coding models.
- Area MT in the visual cortex appears specialized for the computation of motion. Area MST, one stage beyond this, appears specialized for computing global motion percepts such as optic flow. Microstimulation of neurons in these areas can bias the percept of motion towards the cells preferred direction.
is the establishment of a motion percept by instantaneously changing the position of an object. This can occur over fairly large distances. The effect is utilized liberally to decorate buildings in Reno and Las Vegas.
is the global pattern of motion produced on the retina, typically due to ego-motion (self-motion) through the world. It is thought to be used for navigation, balance, and inferring ones direction of heading.
An important parameter that can be inferred from optic flow is time-to-contact: If the optic flow is rapidly expanding about the center of gaze, it means watch out!
Structure from motion
- The kinetic depth effect refers to the recovery of 3D shape information from motion.
- Biological motion, or so-called Johansson dots, is a demonstration of the strong top-down influence on the perception of motion. A small number of moving dots are sufficient to trigger a compelling percept of the movement of people or animals.
are searching eye movements, characterized by quick flicks (movement) of the eye. These movements are conjunctive. They can reveal subconscious or semi-conscious processes.
There is some evidence that cells in the LGN are suppressed in activity during saccades. Also, there appears to be a collapse of perceived visual space prior to a saccade.
There must exist some neural compensation for eye movements via efference copy in order to give the sense of perceptual stability, but the exact mechanism is unknown.
Without saccades, or fixational eye movements, the world would go gray and disappear.
Smooth pursuit eye movements/VOR occur involuntarily.
Eye movements during reading and also during driving reveal sophisticated strategies for gathering information.