 When we hear even a very small intensity sound coming from any direction, immediately we know which side is it coming from, isn't it? Do you know how do we localize from where is sound coming? In a horizontal plane is it coming from left right or if it is in the center, is it in vertical plane that is up or down or in antropostere plane whether it is coming from front or back. The localization of sound in horizontal plane can be done because the sound coming from left or right will reach different ears at different times. Say suppose if it is coming from left side it will reach left ear first compared to right ear. The time difference in reaching both ears is known as inter-oral time difference. Similarly the intensity or loudness of the sound which reaches both ears will also vary because there will be some loss of energy in the longer path. This difference in intensity of sound reaching both ears is known as inter-oral level difference or inter-oral intensity difference. Ok, so now what? See the information from both ears will travel along its pathway to reach cortex. Now in the pathway at the level of the superior olivary complex there is crossing over of information. So each superior olivary complex receives information from both ears. So here we have certain connections to detect the time difference and intensity differences. Actually medial superior olivary nucleus has connections which respond to the time difference and lateral superior olivary nucleus has connections which respond to the intensity difference. To understand this let's assume that the sound of a horn comes from left side. So obviously it will reach left ear first, isn't it? Now from here to superior olivary nucleus that is from left side to left side it will take less time compared to from right side to left side superior olivary nucleus. Now you see neurons in superior olivary nucleus fire only when information from both ears reaches to them together that is at the same time. Is it happening here? No. But now look at the other side. Information from right ear will reach right superior olivary nucleus early compared to that from left ear. So same information is reaching the other side with a delay. So the early reaching of sound to the left ear is compensated by longer line in reaching to the right superior olivary nucleus, isn't it? So in case the information from right and left side reaches this nucleus together the neurons here will fire. So this line will work for a particular time difference. But for more time difference say if information is coming from here more from left there is other line. So there are multiple lines with different length of path so that they reach superior olivary nucleus at different times. So the connections are such that action potential will reach together to the neurons if there is a certain inter-oral time difference. These nuclei in superior olivary nucleus are known as co-incidence detectors. Now this representation of time difference of any sound travels up to the cortex. So there are representations of time difference in the cortex as well that's how we differentiate between sounds in horizontal plane that is whether they are coming from left or right side. However suppose you walk in a room where there is a continuous tone for this kind of sound in the beginning for a split second there will be some time difference but afterwards it will be continuously present for both ears without any time difference. So if we miss that time difference in the beginning we will not be able to detect from where the sound is coming from isn't it? Then how do we localize a continuous tone? See same sound reaches to different ears in different phase something like this. Now there are neurons which fire to different phase of the waves. This we have discussed as frequency theory in another video the link for which we have given in description section below. So depending on the pattern of responses from different ears based on the phase localization of sound is done for a continuous tone but again this mechanism of phase locked response of neurons from ear doesn't work for a very high frequency sound. So hearing becomes difficult to distinguish between different phases since there will be hardly any phase difference between them. So there also exists another mechanism where the neurons detect interaural level difference or interaural intensity difference. So this IID mechanism works very nicely for high frequency sounds. So see if sound is from one side it will reach closer ear with far more intensity than the far ear. And as you might be knowing that loudness is coded as frequency of action potentials in aprons so obviously frequency of action potentials coming from one ear will be different compared to that coming from other ear. Now these IID neurons are stimulated by information from ipsilateral ear and inhibited by information from pontrilateral ear. So if sound is coming from here and arbitrary if we suppose 10 action potentials reach from left ear to left superior olivary nucleus while 4 action potentials reach from right ear. Now since these neurons are excited by ipsilateral ear and inhibited by pontrilateral ear we can say that the level of excitation of IID neurons is 10 minus 4. Now suppose sound is coming from here frequency of action potentials will decrease in left ear while they will increase in right ear. So now maybe 8 action potentials reach from left side and 6 action potentials from right ear. So see level of excitation has decreased. Now remember that the firing rate of IID neurons depends on the level of their excitation. More the excitation or we can say more the intensity difference between the ears more the firing rate and as intensity difference becomes same or intensity in pontrilateral ear becomes more they will stop firing. So till now we talked about how we localize sound if sound is coming from one or the other side in horizontal plane. But how do we localize whether sound is coming from front or back or from up or down? Well the shape of the pinna and convalescence in pinna change the quality of the sound that reaches inner ear depending on where is it coming from and hence further processing occurs accordingly. So basically pinna helps in localization of sound in vertical and anthropocytic plane. Well thanks for watching the video if you liked it do like and share the video and don't forget to subscribe to the channel Physiology Open. Thank you.