What is an "Alternative Sensation"? As far as I know, alternative sensation it's a sensation to distract people from the original stimuli, for example when you have to get an injection, the nurse will pinch you in the other arms so your brain is distracted and will not focus on the original pain in the arms that's got the injection. But I also read here, alternative sensation refers to the brains that can process stimuli from multiple senses, for example when the eyesight is bad, the ears will take up and boost the visual system to help and stimulate the visual system (Visual + Auditory) . Which one is exactly an 'alternative sensation'? Also, what is Time-Frequency in alternative sensation ? I would appreciate any references too.

"Alternative sensation" is not specific terminology in neuroscience - it is just the noun "sensation" with a descriptive adjective "alternative." It just means that it is referring to a different sensation than whatever the "primary" one is.

Knowing what this term means in a given context depends entirely on that context, and may not have any relationship to other contexts.

Time-frequency refers to how something varies in time and frequency. In the context of stimuli, for example, an auditory stimulus can have different "tones" or "notes": those are different sound frequencies, and those frequencies can happen in sequence. To describe a sound visually, one will often present a 'heat map' showing when (time) and at what frequency that sound has energy. Touch stimuli can also have a time-frequency representation, where the frequency indicates the vibration speed of the touch.