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Psyc 2071: Perception & Cognition - 2017

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PSYC2071: Perception & Cognition - 2017

Perception assignment

Question 1

1.1

1.1a

In order to determine the minimum intensity of light detectable by human observers, it is necessary for participants to sit in a dark room for 30 minutes prior to undergoing the experiment. The reason being; individuals will experience pigment bleaching in the retina as a result of exposure to light. Decreased sensitivity due to pigment bleaching is experienced when the eye is exposed to bright light which is then removed; individuals are momentarily blinded by this change in environment. Therefore, in order to avoid the effects of pigment bleaching and enhance sensitivity to light, dark adaptation is required for the recovery of chromophore and opsin receptors. Cone pigment regeneration requires up to six minutes to regenerate, whereas, the rejuvenation of rod pigments takes approximately 30 minutes or more to regenerate. Therefore, participants will undergo dark adaption in a darkened room for 30 minutes to allow for maximum sensitivity to light prior to the experiment.

1.1b

The periphery is the most effective place to present the flash, as rod cells are more sensitive to dim light and are located here. The fovea has a high concentration of cone cells; responsible for colour vision, thus, presenting the flash to the observer’s fovea would be a less effective. Additionally, a stronger activation of individual bipolar cells occurs when a single diffuse bipolar cell is created from convergence in the peripheral retinal pathway. These single bipolar cells with higher convergence reach the intensity threshold earlier through the summation of stimulation. The summation of stimulation is optimal for light vision, and increases the observer’s sensitivity to light. Therefore, the periphery should be used to determine minimal light detection in the human eye.

1.1c

In order to reliably detect the minimum intensity of light, the receptor response must be at its maximum intensity. As rod spectral sensitivity peaks at approximately 498nm, a monochromatic spectral distribution of 500nm is most appropriate for the naked eye.

1.1d

The duration of the flash will be most visible when presented for 0.1 seconds. As mentioned above, dim light is most effectively detected when presented in the periphery. The periphery visual field contains Magno (parasol) ganglion cells which are responsible for processing motion through detecting transient flashes of light at peak transference rates. Therefore, due to the presence of Magno ganglion cells in the periphery, a flash duration of 0.1 seconds will be most effective in determining the minimum light intensity detectable in the human eye.

1.2

1.2a

This subjective approach may impact the accuracy of results when determining participant detection of minimum light in a number of ways. Firstly, participants may have lower confidence levels in their ability to answer and thus be subject to the Hawthorne effect. For example, Observer A may not be focused and therefore, answer that they had been presented the flash when they had not because they believe that is what the experimenter wants from them. This measurement would lead to an increased level of false positives. Results are thus subject to a range of phenomena such as observer bias and the Hawthorne effect. Alternatively, Observer B may report having not seen the flash when they had, and therefore scores would reflect higher levels of false negatives.

The accuracy of this method is poor in measuring participant sensitivity to light as the results rely too heavily on the willingness of observers to report seeing a flash, potentially allowing for too much error in the results.

1.2b

A way to rectify potential observer bias would be to change the instructions from telling the observer each trial consists of a flash, to telling observers that light will flash during certain trials and to respond yes if they witness it. This different method allows for less experimenter interference and decreased pressure on observers. This method will also allow experimenters to measure response outcomes, based on how closely observer’s respond to each flash of light to the actual event.

Question 2

2.1

2.1a

Luminance is the product of illumination and reflectance. With high illumination there

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