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Psy 2071 - Perception & Cognition - 2017

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PSYC2071: Perception & Cognition - 2017

Perception assignment

Question 1



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.


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.


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.


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.



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.


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



Luminance is the product of illumination and reflectance. With high illumination there must be high luminance. Therefore, if both the upper and lower surfaces had 100% reflectance, then the upper square has higher luminance than the lower square. This is due to the upper surface having higher illumination as it is closest to the illumination source, creating higher luminance than the lower surface, which is shadowed, and therefore, as illumination is not as strong, it has lower luminance.


The brain interprets luminance as the result of two factors; illumination and reflectance, with luminance being what the eye receives. Illumination is the perceived amount of light which projects onto an object’s surface and reflectance is the perceived amount of reflectance which is being projected off an objects surface. In the case of the image above, if levels of luminance are equal between the upper and lower squares, then illumination has increased in an inversely proportional manner to reflectance. Due to the layout of the image, where the upper surface seems to be receiving more illumination than the lower surface, our brain automatically perceives the lower surface as having higher reflectance, therefore, appearing lighter to our eyes. Therefore, the relationship between illumination and reflectance manipulates perceived luminance.



Linear perspective allows individuals to judge the distance between objects by creating an illusion of converging parallel lines on a flat surface. The use of the molecular cue, linear perspective, generates an illusion that the upper person is further away since they are higher in the visual field compared to the lower person, thus making us perceive the upper person as bigger. This explains how the retinal size of the upper person is smaller compared to the lower person.


Perception of depth is the product of the relationship between distance, physical size and retinal size. Distance refers to the length between two or more objects, physical size refers to the actual size an object, and retinal size refers to how large an image projects onto the retina. The molecular cue, linear perspective, used in the image above uses converging parallel lines to cause observers to assume distance between two objects. If the distance between two objects is assumed, inferences are made on the size of an object. Therefore, although the people in the image cover the same area on the retina, we assume that the upper person is further away. For the two people to be identical in retinal size, physical size of the upper person must be greater. Therefore, for people to accurately perceive depth, they use the relationship of distance, physical size and retinal size.  


Illumination and distance:

These factors are regarded as being the external influences on the property object.

Reflectance and physical size:

Reflectance and physical size are considered as being the constant perceived properties of an object.

Luminance and retinal size:

Luminance is parallel with retinal size as these components are both dependent on respective cues for accurate perception.

By altering quantities of illumination or distance, our brain reinterprets the reflectance or physical size of the object in order to match the perception of luminance or retinal size.

Question 3


[pic 1]


Ball #1 represents occlusion as it is slightly in front of Ball #2, thus, giving the perception of Ball #1 being closer to the foreground than the ball behind it (Ball #2).

The molecular cue involving all three balls represents retinal size, as Ball #1 appears to be slightly larger in the image than Ball #2 and significantly larger in size than Ball #3.

Retinal position communicates distance and is represented best by Ball #3 as it is higher up in the visual field, thus, giving the perception that it is further away than Ball #1 and #2.


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